I'm new to this as well, so take my thoughts with a large salt vampire.
I ran into this with my new assignment, and I think that TBC definitely means, we're going to continue this thread, I've tagged the folks who I'd like to respond, so let's keep on playing and keep the ball rolling.
Having said that, I'm not sure how to END a thread. I had one to end today so I put "Tag: None" in the subject line, because that's what my specific ship does, and at the end I put something like "End of Scene", because we had reached the natural conclusion between the two characters, and everyone else had moved on.
It will be interesting to see what the more experienced folks have to say.