The 1.7.x branch is not being worked on actively by any developers that I know of. Damian is probably the "official" maintainer of that branch. Nothing much happens here.
BRANCH-1-8 is our "official" stable branch and all bugfixes found are committed to that branch. Commit rate on this branch is slowing down as the obvious bugs have been squashed (hopefully).
BRANCH-1-9 is supposed to be in a feature freeze getting ready for release, but the freeze doesn't seem to be respected anymore. Furthermore, the Tiki leadership has decided to rewrite (before the 1.9 release) the libraries that may or may not have licensing problems when distributed with Tiki. It appears that the so-called "Release Candidate" stage is really more like a beta stage. Final release seems to be a long way off.
HEAD is where all new features are committed since BRANCH-1-9 was supposed to be frozen.
I'm not sure I really answered your questions, but I hope this helps.