>Um, what's the universe on the mainline?

Just at a WAG, there are two choices:

1) Us, here-and-now, act as the mainline, since this is the meta-reality where the Eyrie continuum of universes is actually produced.

2) The UF-Author-Prime universe where the authors who jump into the void in UF4 come from. In that case, UF-Prime is a branch off UF-Author-Prime, which is a subbranch of our timeline...

If there's a reason Time Lords are a little funny in the head, it's because they think about stuff like this all the time. ;)

--
Mr. Fnord, author of the bestselling book How To Violate Spacetime For Your Personal Amusement