Subsector

SSECTORS (an abbreviation for subsector) is the name of a WAD lump which is a component of a level. It is normally generated automatically from other data for the level using a node builder tool.

The node builder creates subsectors out of the sectors, dividing any sector that is not a convex polygon into two or more subsectors that are. Subsector entries are referenced from the nodes lump.

Subsector structure
The subsectors and segs are both stored in sequential order. That is, the seg numbers in subsector 0 are 0 through N0 - 1 (where N0 is the subsector 0 seg count), the seg numbers in subsector 1 are N0 through N0 + N1 - 1 (where N1 is the subsector 1 seg count), and so on.

The node builder assures that all segs in a subsector face the same sector. At level load time, the sector number that the subsector is a part (or all) of is determined by looking at the first seg and this sector number is saved in the internal subsector data structure.

Source

 * Unofficial Doom Specs