Like, I could have cats A and B and C but someone could never make a post in C because it is a "sub-cat only".

I guess similarly the ability to say "main cat only".

Specific situation is a hang gliding / paragliding site. The main cats will be HG or PG with a sub-cat for the type of launch method used. I currently would have to make 12 cats to have 2 mains and 5 subs under each, or 7 total and hope folk can figure it out on there own, but they could also check only one of the launch method types leaving the world to guess if it was HG or PG.

There you go!