Node

NODES 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 nodes lump constitutes a binary space partition of the level. It is a binary tree that sorts all the subsectors into the correct order for drawing. Each node entry has a partition line associated with it that divides the area that the node represents into a left child area and a right child area. Each child may be either another node entry (a subnode), or a subsector on the map.

Node structure
Each of the two bounding boxes describe a rectangle which is the area covered by each of the two child nodes respectively. A bounding box consists of four short values (top, bottom, left and right) giving the upper and lower bounds of the y coordinate and the lower and upper bounds of the x coordinate (in that order).

The type of each child field is determined by its sign bit (bit 15). If bit 15 is zero, the child field gives the node number of a subnode. If bit 15 is set, then bits 0-14 give the number of a subsector.

Source

 * Unofficial Doom Specs