This file is only used by tactical saves.
Always 1250 bytes. Amount used vary depending on the size of the map.
It is a bitfield for fire and smoke, map tiles in order west to east, north to south, top to bottom. Least significant bit is first, most significant bit last.
If a bit is set to 1, then the tile will have a corresponding record in SMOKREF.DAT. Note that any edits to this file are ignored by the game, making it redundant - Most likely it's contents are redefined in memory each turn.