Backspacing offset

They go hand in hand

Backspace is distance from wheel lip/bead to wheel mount surface. Offset is the distance from the wheel width centerline to mounting surface.

8" wheel with the mounting area right in the middle has 4" of backspace and 0 offset

Move the backspace to 3.5" then the offset is -.5"

https://www.rsracing.com/tech-wheel.html