These are generic transition tiles

Filenames consist of:

ABCD - different terrain kinds:
 A - top left terrain
 B - top right terrain
 C - bottom right terrain
 D - bottom left terrain

FFFF - terrain kind:
 F - flat (plain)
 B - bumpy (mountaineous)
 W - waterbed (likely to get removed)

NCNC - transition edge:
 N - none
 C - contrast (sharp)
 S - soft (smooth)