For a diagram that easily explains the NWI map codes, please see the reference diagrom provided below.

                    WETLANDS AND DEEPWATER HABITATS CLASSIFICATION


SYSTEM              SUBSYSTEM        CLASS                     SUBCLASS

                                  |- RB=Rock Bottom            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- UB=Unconsolidated Bottom  1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |
                |-- 1=SUBTIDAL----|- AB=Aquatic Bed            1=Algal
                |                 |                            3=Rooted Vascular
                |                 |                            5=Unknown 
                |                 |                              Submergent
                |                 |
                |                 |- RF=Reef                   1=Coral
                |                 |                            3=Worm
                |                 |
                |                 |- OW=Open Water/Unknown Bottom (used on older
                |                                                  maps)
M=MARINE--------|
                |
                |
                |                 |- AB=Aquatic Bed            1=Algal
                |                 |                            3=Rooted Vascular
                |                 |                            5=Unknown 
                |                 |                              Submergent
                |                 |
                |                 |- RF=Reef                   1=Coral
                |-- 2=INTERTIDAL--|                            3=Worm
                                  |
                                  |- RS=Rocky Shore            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- US=Unconsolidated Shore   1=Cobble-Gravel
                                                               2=Sand
                                                               3=Mud
                                                               4=Organic

 

SYSTEM              SUBSYSTEM        CLASS                     SUBCLASS

                                  |- RB=Rock Bottom            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- UB=Unconsolidated Bottom  1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |
                |-- 1=SUBTIDAL----|- AB=Aquatic Bed            1=Algal
                |                 |                            3=Rooted Vascular
                |                 |                            4=Floating 
                |                 |                              Vascular
                |                 |                            5=Unknown 
                |                 |                              Submergent
                |                 |                            6=Unknown Surface
                |                 |
                |                 |- RF=Reef                   2=Mollusc
                |                 |                            3=Worm
                |                 |
                |                 |- OW=Open Water/Unknown Bottom (used on older
                |                                                  maps)
E=ESTUARINE-----|
                |                  
                |
                |                 |- AB=Aquatic Bed            1=Algal
                |                 |                            3=Rooted Vascular
                |                 |                            4=Floating 
                |                 |                              Vascular
                |                 |                            5=Unknown 
                |                 |                              Submergent
                |                 |                            6=Unknown Surface
                |                 |
                |                 |- RF=Reef                   2=Mollusc
                |                 |                            3=Worm
                |                 |
                |                 |- SB=Streambed              3=Cobble-Gravel
                |                 |                            4=Sand
                |                 |                            5=Mud
                |                 |                            6=Organic
                |                 |
                |                 |- RS=Rocky Shore            1=Bedrock
                |                 |                            2=Rubble
                |                 |
                |-- 2=INTERTIDAL--|- US=Unconsolidated Shore   1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |
                                  |- EM=Emergent               1=Persistent
                                  |                            2=Nonpersistent
                                  |
                                  |- SS=Scrub-Shrub            1=Broad-Leaved
                                  |                              Deciduous
                                  |                            2=Needle-Leaved
                                  |                              Deciduous
                                  |                            3=Broad-Leaved 
                                  |                              Evergreen
                                  |                            4=Needle-Leaved
                                  |                              Evergreen
                                  |                            5=Dead
                                  |                            6=Indeterminate
                                  |                              Deciduous
                                  |                            7=Indeterminate
                                  |                              Evergreen
                                  |
                                  |- FO=Forested               1=Broad-Leaved
                                                                 Deciduous
                                                               2=Needle-Leaved
                                                                 Deciduous
                                                               3=Broad-Leaved
                                                                 Evergreen
                                                               4=Needle-Leaved
                                                                 Evergreen
                                                               5=Dead
                                                               6=Indeterminate
                                                                 Deciduous
                                                               7=Indeterminate
                                                                 Evergreen



 
SYSTEM              SUBSYSTEM        CLASS                     SUBCLASS

                                  |- RB=Rock Bottom            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- UB=Unconsolidated Bottom  1=Cobble-Gravel
                                  |                            2=Sand
                |--1=TIDAL--------|                            3=Mud
                |                 |                            4=Organic
                |                 |
                |                 |-*SB=Streambed              1=Bedrock
                |                 |                            2=Rubble
                |                 |                            3=Cobble-Gravel
                |--2=LOWER        |                            4=Sand
                |    PERENNIAL----|                            5=Mud
                |                 |                            6=Organic
                |                 |                            7=Vegetated
                |                 |
                |                 |- AB=Aquatic Bed            1=Algal
R=RIVERINE------|--3=UPPER        |                            2=Aquatic Moss
                |    PERENNIAL----|                            3=Rooted Vascular
                |                 |                            4=Floating 
                |                 |                              Vascular
                |                 |                            5=Unknown 
                |                 |                              Submergent
                |--4=INTERMITTENT-|                            6=Unknown Surface
                |                 |
                |                 |- RS=Rocky Shore            1=Bedrock
                |                 |                            2=Rubble
                |                 |
                |                 |- US=Unconsolidated Shore   1=Cobble-Gravel
                |--5=UNKNOWN      |                            2=Sand
                |    PERENNIAL----|                            3=Mud
                   (used on older |                            4=Organic
                    maps)         |                            5=Vegetated
                                  |
                                  |-**EM=Emergent              2=Nonpersistent
                                  |
                                  |- OW=Open Water/Unknown Bottom (used on older
                                  |                                maps)
                                  |-*STREAMBED is limited to TIDAL and
                                  | INTERMITTENT SUBSYSTEMS, and comprises 
                                  | the only CLASS in the INTERMITTENT SUBSYSTEM.
                                  |
                                  |-**EMERGENT is limited to TIDAL and LOWER
                                  | PERENNIAL SUBSYSTEMS.




SYSTEM              SUBSYSTEM        CLASS                     SUBCLASS

                                  |- RB=Rock Bottom            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- UB=Unconsolidated Bottom  1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |
                |-- 1=LIMNETIC----|- AB=Aquatic Bed            1=Algal
                |                 |                            2=Aquatic Moss
                |                 |                            3=Rooted Vascular
                |                 |                            4=Floating 
                |                 |                              Vascular
                |                 |                            5=Unknown 
                |                 |                              Submergent
                |                 |                            6=Unknown Surface
                |                 |
                |                 |- OW=Open Water/Unknown Bottom (used on older
                |                                                  maps)
L=LACUSTRINE----|
                |
                |
                |                 |- RB=Rock Bottom            1=Bedrock
                |                 |                            2=Rubble
                |                 |
                |                 |- UB=Unconsolidated Bottom  1=Cobble-Gravel
                |                 |                            2=Sand
                |                 |                            3=Mud
                |                 |                            4=Organic
                |                 |
                |                 |- AB=Aquatic Bed            1=Algal
                |                 |                            2=Aquatic Moss
                |                 |                            3=Rooted Vascular
                |                 |                            4=Floating
                |-- 2=LITTORAL----|                              Vascular
                                  |                            5=Unknown
                                  |                              Submergent
                                  |                            6=Unknown Surface
                                  |
                                  |- RS=Rocky Shore            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- US=Unconsolidated Shore   1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |                            5=Vegetated
                                  |
                                  |- EM=Emergent               2=Nonpersistent
                                  |
                                  |- OW=Open Water/Unknown Bottom (used on older
                                                                   maps)



SYSTEM              SUBSYSTEM        CLASS                     SUBCLASS

                                  |- RB=Rock Bottom            1=Bedrock
                                  |                            2=Rubble
                                  |
                                  |- UB=Unconsolidated Bottom  1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |
                                  |- AB=Aquatic Bed            1=Algal
                                  |                            2=Aquatic Moss 
                                  |                            3=Rooted Vascular
                                  |                            4=Floating
                                  |                              Vascular
                                  |                            5=Unknown 
                                  |                              Submergent
                                  |                            6=Unknown Surface
                                  |
                                  |- US=Unconsolidated Shore   1=Cobble-Gravel
                                  |                            2=Sand
                                  |                            3=Mud
                                  |                            4=Organic
                                  |                            5=Vegetated
                                  |
                                  |- ML=Moss-Lichen            1=Moss
                                  |                            2=Lichen
                                  |
P=PALUSTRINE----------------------|- EM=Emergent               1=Persistent
                                  |                            2=Nonpersistent
                                  |
                                  |- SS=Scrub-Shrub            1=Broad-Leaved
                                  |                              Deciduous
                                  |                            2=Needle-Leaved
                                  |                              Deciduous
                                  |                            3=Broad-Leaved 
                                  |                              Evergreen
                                  |                            4=Needle-Leaved
                                  |                              Evergreen
                                  |                            5=Dead
                                  |                            6=Indeterminate
                                  |                              Deciduous
                                  |                            7=Indeterminate
                                  |                              Evergreen
                                  |
                                  |- FO=Forested               1=Broad-Leaved
                                  |                              Deciduous
                                  |                            2=Needle-Leaved
                                  |                              Deciduous
                                  |                            3=Broad-Leaved
                                  |                              Evergreen
                                  |                            4=Needle-Leaved
                                  |                              Evergreen
                                  |                            5=Dead
                                  |                            6=Indeterminate
                                  |                              Deciduous
                                  |                            7=Indeterminate
                                  |                              Evergreen
                                  |
                                  |- OW=Open Water/Unknown Bottom (used on older
                                                                   maps)



                                   MODIFIERS

                                  |- A=Temporarily Flooded
                                  |- B=Saturated                      
                                  |- C=Seasonally Flooded
                                  |- D=Seasonally Flooded/Well Drained
                                  |- E=Seasonally Flooded/Saturated 
                                  |- F=Semipermanently Flooded
                |--Non-Tidal------|- G=Intermittently Exposed
                |                 |- H=Permanently Flooded
                |                 |- J=Intermittently Flooded
                |                 |- K=Artificially Flooded 
                |                 |- W=Intermittently Flooded/Temporary (used on
                |                 |                                    older maps) 
                |                 |- Y=Saturated/Semipermanent/Seasonal (used on
                |                 |                                    older maps)
                |                 |- Z=Intermittently Exposed/Permanent (used on
                |                 |                                    older maps)
WATER REGIME----|                 |- U=Unknown
                |                 
                |                  
                |                    
                |                 
                |                 |- K=Artificially Flooded
                |                 |- L=Subtidal   
                |                 |- M=Irregularly Exposed  
                |                 |- N=Regularly Flooded 
                |--Tidal----------|- P=Irregularly Flooded
                                  |-*S=Temporary-Tidal   
                                  |-*R=Seasonal-Tidal
                                  |-*T=Semipermanent-Tidal
                                  |-*V=Permanent-Tidal
                                  |- U=Unknown
                                  | 
                                  |-*These water regimes are only used in 
                                  |  tidally influenced, freshwater systems.

                                  

                                  |- 1=Hyperhaline
                                  |- 2=Euhaline
                |--Coastal        |- 3=Mixohaline (Brackish)
                |  Halinity-------|- 4-Polyhaline
                |                 |- 5=Mesohaline
                |                 |- 6=Oligohaline
                |                 |- 0=Fresh
                |
                |
                |
WATER CHEMISTRY-|
                |                 |- 7=Hypersaline
                |--Inland         |- 8=Eusaline
                |  Salinity-------|- 9=Mixosaline
                |                 |- 0=Fresh
                |
                | 
                |
                |
                |--pH Modifiers   |- a=Acid
                   for all        |- t=Circumneutral
                   Fresh Water----|- i=Alkaline




SOIL------------------------------|- g=Organic
                                  |- n=Mineral




                                  |- b=Beaver
                                  |- d=Partially Drained/Ditched
SPECIAL MODIFIERS-----------------|- f=Farmed
                                  |- h=Diked/Impounded
                                  |- r=Artificial Substrate
                                  |- s=Spoil
                                  |- x=Excavated

U = Uplands

(Source: US FWS)

 See also the USFWS Wetland MApper Interactive Website & http://www.fws.gov/data/datafws.html