Htflags

A per-actor property, this controls special flags for sprites. The values are:

1 - Generates a shadow. See spriteshadow.
2 - Turns palette 6 whenever night vision goggles are used. See spritenvg.
4 - The sprite's shade is unaffected by the shade of its sector. See spritenoshade.
8 - Is a projectile.
16 - Prevents teleporting. The sprite is not entered into the decal deletion queue.
32 - Is an enemy. See useractor.
64 - Sprite will be insusceptible to floor palette of sector containing this sprite. See spritenopal.
128 - Sprite will not be run through event code.
256 - Sprite will not emit hardcoded Polymer-based lights.
512 - called SPRITE_USEACTIVATOR in actors.h, internal?
1024 - called SPRITE_NULL in actors.h, internal?
2048 - Sprite will skip x/y "clipmove()" . This skips objects checking for collisions with other actors to increase performance. Useful for particle effects.

htflags is a bitfield, so add together the values you want and then set htflags to the sum.
This member can also be set using the per-sprite form of the spriteflags command.