Sector

From ZDoom Wiki

A sector is an enclosed area made up of lines in the map. Sectors are referenced by the Sidedefs linked to the enclosing Linedefs. Sectors must be closed areas, as in all sidedefs that reference them must make up one or more closed shapes. Note that having unclosed sectors is possible, and can be used for a few special effects, but can cause unpredictable results (including crashes) if used incorrectly.

As stated, a sector need not enclose only one area of the map. To avoid redundancy, two separate areas of the map could be defined by one sector, thus copying all the properties of one area to the other. However, be aware that the engine treats these separate shapes as the same sector. In particular, sounds entering one area of the sector will be heard by enemies in the other. This can be used intentionally to allow sounds generated in one area of the map to alert enemies in another, unconnected area.

Specification

Bytes	Data Type	Description	Notes
0-1	Signed Short	Floor height
2-3	Signed Short	Ceiling height
4-11	Char	Floor texture
12-19	Char	Ceiling texture
20-21	Unsigned Short	Light level	Vanilla Doom rounded the light level to the nearest multiple of 8, ZDoom shows unique light levels for all 256 values
22-23	Unsigned Short	Sector special	Sector specials
24-25	Unsigned Short	Linedef / Thing tag	Action Specials