There really aren't sub categories. This isn't reddit. There is just the main site. You can filter out to only see Show, Ask, and Jobs (listed on the top bar), but other than that there is nothing (also those are really just filters, not sub categories, as show, ask, and jobs will show up on the main page).
Not 100% sure, but I don't think there is any other than the "ask" and "show" which are official categorized (into each section) by hand and out of the submitters's control.