I can take a stab at the last bit. I'd guess that there is a "tag" which we don't see. If a thread is started as "BS:", it gets set automatically, on starting the thread but otherwise it isn't set. The site admins can later change the setting of this "tag" if need arises.

In other words, it is initially an honour system but can be over-ridden at any time.

As for the rest, and I'll in no way argue about it being right for MC, the common approach is to have forums (or rooms) and categories on top of that. That way, you can say have a general approach, eg. music or subdivide music as well as divide into rooms with a moderator. How you choose to display is another matter, most go for the "room" aproach but perhaps the split view here is better for MC, personnally, I like it.

I'm feeling nervous here as I don't want to be thought of again as putting out rows or criticisms when I'm only trying to answer a question but if you are interested, try my search (here. Just put "Jon" in as a user name and play around with changing the "search in" list boxes (the second takes priority) before hitting "search" and you should get (thread wise) anything I posted, anything I posted musical or even find anything I posted on musical instruments.

Jon (only trying to share information and I think that bit illustrates at least part of a commonly used structure)