Skater,
as they said above, BS is the distance from the back lip of your rim to the mounting surface. the amount of BS will be affected by a few things ie.. clearance up front for turning and if your running a sway-bar out back. I currently have a D-44 on the back of mine and I am running with the sway bar connected. I was about to buy some used 15x8 rims with VERY used 31x10.5 FCIIs and luckily decided to try to mount them before I pulled the trigger. ... The rims had a BS of 4.75 (which is >.5" SMALLER than stock..meaning they would stick out .5" more) and the tire was rubbing on the sway bar to the point they would not turn (on the rear axle). because of MY setup I am limited to 4" or less BS (currently running 16" rims with 3.75" BS with 265/75R16s) Yes even though this tire is only 10.5" wide they still stick out and rub BAD in the back when I stuff them up. I currently have a 5" "lift" and have no issues with rubbing up front.

I plan on running 15x10" rims with 3.75" BS on a 32x11.50. I have been told this will not rub at 5" and am pretty confident with good bumpstops I wont have to trim the fenders...if not Oh well, a cutting I will go!

I have spent at least a month on the whole decision process for type, size, BS, tire ETC... and most of my info has been from the search button here and some trial and error with other peoples stuff (trying on different wheel tire combos)

I hope this helps a little more, Good luck with the eternal "search for better tire/wheel"

XJinch
(bottom pic to show how far they stick out with 16x8 w 3.75"BS)