This file is only used by tactical saves.
Length of file is map size (overall number of tiles) + some 4 bytes at the end.
File contains data about light from mobile sources (soldiers' personal lights). It is recalculated every time a unit moves (does action requiring energy). Together with TERMP.DAT it is full info about current lighting of the map. Actual lighting of tile is lower value from both files. Exact meaning of values is a number that pallette is shifted towards darker colours.