hi, I have a question.

I know with spacers an aftermarket M3 Wheel (5x120, 19") will fit an M5.

What does M3 fitment actually mean? The measurements used for an M3 are the same as an M5 (e39 model).

Could someone fill me in?

Thanks

The "fitment" may be in reference to the wheel offsets.

The "measurements" for an E46 M3 and an E39 are not the same. Wheel sizes may be the same, but definitely not the offsets.