A vertical door is a sector where the floor and the ceiling are at the same height. Opening and closing the door is done with one of the door specials. While opening, the door's ceiling gradually moves up to 4 map units below the lowest neighboring ceiling (for this reason, two different door sectors cannot touch).
Vertical doors can be either "manual" or "remote". A manual door is when the door sector is the activating linedef's back sector, rather than a tagged sector. A remote door is any door activated from its tag. The main gameplay difference between both is that a manual door can be retriggered repeatedly while it is moving, forcing an opening door to close or a closing door to reopen; while a remote door cannot be retriggered until it has completed its movement.