<slotID> is the index of the slot which will be used to draw a model. Legal values are 0 through 254.
The purpose of the guniqhudid command is that models drawn to the screen via rotatesprite do not have a sprite id like all other sprites in the game world. The engine needs some sort of ID so that it knows where and what to draw.
This command will allow you to select a different slot so that any HUD models you are drawing can store their animation state independently, without conflict. By default it uses the first slot, 0. If you need several animating HUD models, you should switch to another slot, draw the model with rotatesprite, then switch back using
guniqhudid is short for "g (game? gun?) unique HUD ID".