back spacing is mesured from the back of the rim to the mounting flange.

a 6" rim with 4 1/2" backspacing would set inward 1/2" more than one with 4" backspacing, and would only leave 1 1/2" from the mounting flange to the outside of the rim instead of 2".

i guess this might be the difference on some rims whether they fit disc brakes or not?