You can have 2 levels of Categories, we’re currently only using 1 level. So you can do something like Having a “Core” category that has “Committers”, “Devel”, etc inside it. Discourse does not let you arbitrarily nest categories beyond 2 levels though, and the recommendation at that point is to start using tags instead.
We should be able to edit these if needed I think. They won’t apply to existing users, but if we want to change the default settings for new users I believe we can.