Lifts, also internally known as plats (short for platforms), are a class of sector thinkers which affects the floor motion of a sector in ways typically more complicated than simple floor actions which raise or lower to a given height.
The most common type of platform is the simple lift. In Doom, lifts move from their starting height down to the lowest surrounding floor height (if no such sector exists, no movement takes place). They can move at speeds of 4 or 8 map units per tic. When moving at the speed of 8 units/tic, they are described as "blazing" platforms. All such lifts wait for a time period of 3 seconds before returning to their original starting height. In editors, such lifts are sometimes described in terms of their motion sequence as "down-wait-up-stay" lifts.
Strife, Hexen, and Doom 64 all provide independent implementations of a similar platform action which starts at a low height and raises to the height of the highest surrounding floor. These are likewise referred to as "up-wait-down-stay" lifts.
Another type of platform action implements a simple floor raising action which changes the properties of the sector it affects. A "raise to nearest and change" platform action sets the height of its sector to that of the next highest neighboring floor, adapts the floor texture on the first side of the activating switch linedef, and sets the sector special to zero. A "raise and change" platform action raises by a set amount depending on the line trigger type, adapts the floor texture on the first side of the switch, and does not change the sector's special. Both of these types move at a speed of 0.5 unit per tic.
A final type of platform is referred to as "perpetual raise", and forever moves between two different heights unless suspended via a "stop platform" action. This type of platform moves between the lowest and highest surrounding floors, reversing motion whenever the current destination height is reached after a short pause. Whether such a floor initially moves up or down is random. The speed of this type of platform is one unit per tic.
Only 30 platform-type actions can be active at once, regardless of type. If this limit is exceeded the game will bomb out with the error message "P_AddActivePlat: no more plats!". Most source ports remove this limitation.