Wheel width, backspace and offset are all related (if you know 2, you can calculate the 3rd)

How far a wheel sticks out is commonly referred to as "front-face". This is how much of the wheel sticks out past the hub surface and is just the wheel width minus the backspace.

8.5" wide wheel with -6mm offset = 4.5" backspace
8.5 - 4.5 = 4" front-face

9" wide wheel with 0mm offset = 5" backspace
9 - 5 = 4" front-face

Both wheels stick out 4" from the hub.