It's the stator, which is the outside of the can... Well Inside the can just under the skin. On some motors you can change this to change the number of windings in the motor. Novak ballistics for example. Changing the rotor is where I'm not 100% on, but I think it's to do with the tourqe of the motor. Bigger rotor = bigger magnet = more tourqe? I do know if your racing spec the rotor needs to be approved by a local governing racing body ie. ROAR. And even then not sure if your allowed to for stock classes or if it's mod only.