What is synchronous reference frame?

It has been shown that the speed of rotation of the d.q axes can be arbitrary although there are three preferred speeds or reference frames as follows:

(a) the stationary reference frame when the d,q axes do not rotate;

(b) the synchronously rotating reference frame when the d,q axes rotate at synchronous speed;

(c) the rotor reference frame when the d,q axes rotate at rotor speed.