Hey, thanks for getting back. Nice to talk to someone in the space, there don't seem to be very many haha.
You've confirmed my suspicions, thanks for a definitive answer. It's a bit of a shame that it's not exposed and alterable, but I'll have to make do with what we're given.
Using tp_mod_usage_addition to perform subtraction did occur to me, I say as much in the sentence you quoted, I was just using tp_mod_usage_subtraction as a fictitious example to highlight the kind of thing I'd like to be able to do.
Either way, I can probably do something hacky like use the heavy weapons team entrenchment ability as a scaffold for transforming a land unit into a flying unit and back again. I gather I'll be abusing things like that to smash together the functionality I want haha