the area where the brake pad wears cuts into the rotor. the very end of the rotor is the original thickness.
volvo requires replacing pads, rotors and sensors as an assembly whenever doing a brake service. do not try and short cut it or do less than required.
yes roy is correct all short cuts will cause you more problems.they also call it dished, the wear of the pads make the center of the rotor dish which causes the edge to get a lip on it.most cars you can turn the rotors and the lip will be gone. on certain cars ex. benz,bmw,saab,audi and a few others the rotors pads and sensors need to be replaced every brake job.if you don't this can cause others issues that will make it more costly in the long run.do it right the 1st time and save $ in the long run along with hours of down time.