A dummy sector is a sector polygon disconnected or at least hidden from the playable area, which is typically used to achieve special effects, notably through use of linedef types that copy properties from neighboring sectors (floor or ceiling movements, light level changes, etc.). Another possible use is ensuring certain effects will work correctly, to avoid problems such as raising sectors not changing to the intended sector type. They can also be used for sound propagation.
Since sectors in the Doom engine can be made of several disconnected polygons, it is possible for a dummy sector to have a zone in the playable area.
Dummy sectors can also be referred to as control sectors when they are used in conjunction with certain linedef types that require use of a dummy sector, such as most implementations of 3D floors and Boom's property transfers. Polyobjects are also normally built in dummy sectors.
While dummy sectors are normally impossible to see, they may appear on the automap when finding a computer area map or equivalent powerup, or because of bugs in the automap sight code. Some source ports try to identify control sectors to prevent them from being displayed on the automap.