The CPR numbers give a hint as to whether a ball will bounce higher or lower on a specific surface, and how much friction there is to either slow the ball or allow it to skid on.

CNN: Tennis interactive: Why clay is so different

When a spinning ball hits a low friction surface such as grass, it will continue to spin.

BBC: SPORT | Tennis | Rules and Equipment | On the rebound

But on a high friction surface like clay, the ball will grip more and change the trajectory of the ball quite significantly.

BBC: SPORT | Tennis | Rules and Equipment | On the rebound