From OpenStreetMap Wiki
|Unrelated to GNIS, this is actually an NHD Feature Type (nhd:ftype)|
|Used on these elements|
|Tools for this tag|
gnis:ftype=* is not part of GNIS. This is likely the result of a bug in older versions of an NHD .shp conversion script. It should be nhd:ftype=* or simply omitted because it's a redundant alternative form of nhd:fcode=*.