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:
sprite_cstat and wall_cstat should have only set the bits which should be clipped. The most important bits are blockable (flag 1), hitable (flag 64 for walls and 256 for sprites), masking wall (flag 16).
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.