Clipmask is a value that tells what kinds of objects should be clipped or not. The lower 16 bits are anded (&) with the cstat of a wall and the higher 16 bits are anded with the cstat of a sprite. Clipmask is used by the getzrange, hitscan, and movesprite commands.
One can calculate a mask using this formula:
The most important bits are blockable (flag 1), hitable (flag 64 for walls and 256 for sprites), masking wall (flag 16).
Pre-defined values =
The game has two predefined clipmasks: CLIPMASK0 and CLIPMASK1. Apparently the first one is used by regular actors, while the second is for projectiles.
CLIPMASK0 = (1*65536)+1 = 1 0001h
This clips to blocking sprites (flag 1) and walls (flag 1).
CLIPMASK1 = (256*65536)+64 = 100 0040h
This clips to sprites that can be hit by (hitscan) weapons (flag 256) and walls (flag 64).
Sample clipmask for hitscan (4294901808) = FFFF 0030h
This clips sprites which have any flags and walls which are one-sided or masking.