Inside actor, state, and event code, the command
sets the htflags value for the current sprite.
Outside of actors, states, and events, the directive
spriteflags <picnum> <value>
sets the same flags per-tile. At game time, this per-tile value is XORed with the per-sprite value, i.e. the setting of one to "true" reverses the other.
For a particular actor tile, the spriteflags directive should precede its actor definition, as well as uses of other sprite* flag modification directives.