Seg

SEGS (an abbreviation for segments) 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 divides linedefs into segs, as necessary, to create the boundaries for subsectors. (The node builder will likely create additional vertexes in order to do this.) The segs are stored in sequential order corresponding to subsector number. The seg entries are referenced from the subsector entries, which are referenced from the nodes lump.

Seg structure
The direction field may be interpreted as a "side flag": direction=0 indicates the seg runs along the right side of the linedef; direction=1 indicates it runs along the left side.

Drawsegs overflow
While constructing a frame, the segs to be drawn are stored for later rendering. In vanilla Doom, they are kept in a static array 256 elements long, and if a frame requires more segs than this, the excess segs are simply not drawn.

Source

 * Unofficial Doom Specs