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, but not many seem to be used.
Structure
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 serves some purpose. Perhaps terrain and alien craft stuff.
For a UFO site, 76/4C is the LOC.DAT reference to zero after winning. It plausibly is always the LOC.DAT reference involved.
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.
The following bytes allow you to use the following equipment only if they are set to 1:
152/98 - Heavy Plasma 153/99 - Heavy Plasma Clip 154/9A - Plasma Rifle 155/9B - Plasma Rifle Clip 156/9C - Plasma Pistol 157/9D - Plasma Pistol Clip 158/9E - Blaster Launcher 159/9F - Blaster Bomb 160/A0 - Small Launcher 161/A1 - Stun Bomb 162/A2 - Alien Grenade 163/A3 - Elerium-115 164/A4 - Mind Probe
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).
All other bytes in this file seem to be set to 0.