Backspacing is the distance from the back of the wheel to the wheel mounting flange. Offset is the distance the wheel mounting flange is moved from the center of the wheel. A positive offset means it is moved toward the outside of the wheel, and negative offset means it is moved toward the inside of the wheel. Since the instructions say you can run a maximum of 5" backspacing, you need to find a wheel with a 0 or negative offset if you want to run a 9" wide wheel.