I've found this sort of channel delineation extraordinarily hard to capture in language, while also being memorable, consistent, and welcoming.
It might be worth trying to do some writing for the Handbook about the meta-level matter of "What to do if you aren't sure where to post," in such a way that exudes a sense of forgiveness.
I'd much rather people post something to a sub-optimal channel, but still post, rather than not post at all, or throw up their hands and just dump everything wherever. (And we've seen both of those failure modes occur here.)