The National Wetland Inventory maps are a set of maps based upon the 1:24000 (7 1/2 minute) scale. They have been around since the mid-1980's with very little update since then. These maps were drawn from "fly overs" performed by the U.S. Fish and Wildlife Service and many have not been "ground truthed". Whatever you see on these maps that is indicated with Cowardin codes appeared to be wet and vegetated at the time of the fly over. It is the investigators responsibility to verify that that the area indicated to be a wetland actually possesses all three characteristics of wetlands.
Follow this link to the Cowardin Codes table. You should be able to follow the drop down menu to find the meaning behind any code you find on an NWI map of Oklahoma.