MISDATA.DAT
This 170 byte file is only used by tactical saves. It stores such information as the craft in use, and which alien artefacts are available for use in combat. Not many bytes have been decoded as yet.
Structure
Many of the later bytes are set to either 0 or 1. Values of 0 mean you can't use certain pieces of alien equipment, though I've yet to test as to which bytes effect which weapons.
X-Com produced equipment can be used even if it hasn't been researched (however, in order to get it into combat, you must build it - and that requires research).
20/14 is the mission descriptor. Possible values:
0: UFO site 1: Terror site 2: Base defense (X-Com base) 3: Base offense (Alien base) 4: Mars part 1 5: Mars part 2
74/4A and 76/4C serve some purpose. Perhaps terrain and alien craft stuff.
77/4D seems to map to the craft in use. I assume this value indexes off somewhere else (eg CRAFT.DAT). It seems to also serve when one of your bases is under attack (BASE.DAT). Most likely it is used when combat is over, as it shouldn't be required for creating a new tactical map.
79/4F is 0 unless the mission is a UFO site, in which case it is 1.