Okay..I ran through all the text files generated into the log folder with a search function on the keyword (ExecSetSlot) and (CheckSlot) and found this:
Slot 9 is used for changing the gauge of Axis/Soviet rail in event 32 and 63. For example, if it is a 4, it is a German Rail, if it is a 3 then a little 1 will show up on the map, and it will be unusable...
Slot 3 is used by an editor function and is used by event 10 to set weather..which changes slot 4.
Slot 4. The value for weather is calculated and placed in slot 4. I plan to dig into event 10 and figure it out more, as it shatters some assumptions I have made about weather in this game.
Slot 5 is used in event 157 Militia reinforcement. It seems to be used to prevent cities from generating more then one militia unit (per hex).
Slot 7 is not set by event, but it is read by event 173 Fort Select Hex and done when the Fortification card is bought.
Slot 2 is not set by event, but is read by event 60, Soviet Oil, and seems to be the supply value of the hex...so is probably set by the supply function.
Slot 0 is used by events 19 + 20 (Minor Orders, Major Orders), 37 + 38 (Placing Forts, Placing Units) and event 173 (Fort Hex Selection). It seems to be a temporary value used to put the result of a calculation.
Slot 1 is used by the editor and is set. I havent found an event that uses it, however. I suspect it is used by the AI or just not used at all.
Which leads me to believe that I can use slot 6 and 8 for myself, and possibly slot 1. And I can use slot 0 for temporary calculations.
“My logisticians are a humorless lot … they know if my campaign fails, they are the first ones I will slay.” – Alexander the Great