Maybe this is patently obvious, but, for the @moderators thinking about categories, I’d just note that forum users will always have two (un-articulated) questions in mind when figuring out where to post something: “which category makes most sense to me?” (context) and “what does a category allow me to do?” (settings, permissions)
So you’ll potentially find as people get their head around the rules / settings applied to each category, they post where it’s useful rather than where they think similar content goes. e.g. “this post really goes in category x, but I’ll put it in category y so that anyone can comment”.
It’s sometimes useful to bear in mind when coming up with categories and rules, IME - and also something to not be surprised by down the track when you see it happening.