When I saw the Categories I thought they were sub-forums as that's typically how other forums operate. But they way they operated didn't make sense -- particularly the fact that multiple "categories" could be selected -- so I largly ignored them and just stuck with "All Forum Topics".
I did a little test, creating a dummy topic and selecting several categories. My test post only showed up in those selected categories -- naturally, it showed up in the "All Forum Topics", as all posts seem to do.
I then edited my post to change some of the categories. Again, my post showed up in only the categories I changed it to, as logically expected.
So this was my "ah ha!" moment: The categories are used as a filtering device! There are no sub-forums; those Categories to the right of the screen are actually filters, only showing posts that have selected the Categories in the original post. So I view them as kind of like a mega-tag, a broad tag filter if you will, while actual Tags are used [or should be used?] to detail or get more specific into the post much like a report drill-down.
Yes, as a young child I had naturally platinum blonde hair...