Sector

A sector is an area referenced by Sidedefs on the Linedefs. Sectors should be closed areas, meaning that all the sidedefs that reference a particular sector should make up a closed shape. Note that having unclosed sectors is possible, and can be used for a few special effects, but you shouldn't really attempt to use them unless you're pretty experienced in level editing since they can cause unpredictable results (including crashes).

Ceiling Flat - This is the name of the flat (texture) used to texture the floor of the sector.

Floor Flat - Same as the ceiling flat, only used on the floor.

Ceiling Height - The height of the ceiling.

Floor Height - The height of the floor.

Lighting - How bright the sector is.

Sector Tag - A number used that allows you to alter the sector's ceiling and/or floor height, lighting level or even flats. This is accomplished by having an "action" linedef somewhere that has the same tag number as the sector(s) to be altered.

Sector Type - A defining property of the sector. These allow you to give a sector some form of damage (for things like lava and slime), have the sectors light level change constantly, enable wind or currents or even push the player in a certain direction. See below.

Sector structure
Sectors are stored in WAD files in the following format:

Sector types
The type field of the sector structure is used to specify lighting, damage, and a few other special effects.
 * There are linedef types that can change the sector type field during gameplay, either to zero or to a copy of another sector's type field. This is how, for example, a safe bridge rises out of a pool of nukage.
 * Damage is halved at skill level 1.
 * Damage is split between health and armor (if any).

Vanilla Doom
In vanilla Doom, the sector type values are mutually exclusive.

Boom
Boom (and compatible source ports) support the Doom sector types, and also provide "generalized" sector types. The latter are based on bit fields, which allows several sector type properties to be independently specified for a sector. Texture change linedef types can be used to switch some or all of these properties dynamically, outside lighting.

Bits 0 thru 4 specify the lighting type in the sector, the same codes that Doom uses are employed (0, 1, 2, 3, 4, 8, 12, 13 and 17 from above).

Bits 5 and 6 set the damage type of the sector, with the usual 5/10/20 damage units per second.

Bit 7, when set, makes the sector count towards the secrets total at game end.

Bit 8, when set, enables the ice/mud effect controlled by linedef 223.

Bit 9, when set, enables the wind effects controlled by linedefs 224–226.