Behaviour in Doom is controlled by using a combination of tags and types applied to linedefs and sectors. Tags are used to relate an action, as specified by a linedef type, to the sector (or sectors) which it will affect.
Normally, a tag is an arbitrary non-zero integer, specified in the tag field of linedef and sector entries.
For example, in the northernmost room of MAP01: Entryway (Doom II), there are four platforms with troopers on them; two of the platforms have switches on their sides facing the center of the room. The switch on the eastern platform is implemented using a linedef type of 103 ("switch once: remote door open and stay open") and a linedef tag of 4. In the northeast corner of the room, the wall is actually one side of a sector with a sector tag of 4. Thus, when the switch is activated using the USE key (spacebar by default), the wall in the northeast corner opens like a door.
The other switch in this room is a linedef with type 102 ("switch once: move floor down to highest neighbor floor height") and a tag of 3. The four platforms all have different sector numbers, but they all have a tag number of 3. Therefore, activating the switch lowers all four platforms.