Okay, it's kinda described
here.
The problem with that is that this is the old, table-based mission generation and e.g. these CultApprehensionEXALT missions have 'useTable: false', which would lead one to guess that this limitation would no longer apply. But apparently it does.
So, this is
very poorly documented at best, and expecting modders to code dive for something that can have a
major impact on script-based mission generation is... not a terribly productive idea.
Am I understanding this right that since e.g. CultApprehension
Lotus has four regions defined, any time the game tries to generate the mission, it tries to pick one of the four that hasn't been used yet, everything beyond that gets ignored, and thus you'll ever get a maximum of four such missions per month, spread across
all four regions? And the actual region distribution pretty much only determines the region-vs-time order of those missions if there are four of them?
If so, that's a
major facet of
script-based (as opposed to table-based) mission generation and very much needs at least a mention in the ruleset reference.