What does the RELENG_4_8_BP represent

[Bill Moran]

I'm familiar with -STABLE, -RELEASE, -CURRENT and the
various RELENG_x_x branches, but I'm not familiar with
the RELENG_x_x_BP branch.

What is the purpose of this?  I couldn't find anything in
the handbook on it.

Once I understand what the purpose is, I'll make a change to
the handbook and submit a PR, since I couldn't find it there
(I'll also add information on the patch branches, as this
seems to be missing as well)

If I'm wrong, and this information _is_ in the handbook,
please point me at it, as it's very possible that the
handbook would benefit from a cross-reference.

-- 
Bill Moran
Potential Technologies
http://www.potentialtech.com