Here is what rotor phasing "is." Whether breaker points, magnetic, whatever, "there is" a trigger that causes the spark to happen. You want this spark to happen when the rotor is aligned with the appropriate tower contact in the cap. If the rotor contact is "between" two cap towers, crossfire, and cap "carbon tracking" will happen.
In a "normal" distributor with mechanical advance, the rotor and mechanical advance move together. So if the rotor is properly phased at one point, movement of the advance mechanism will not change the alignment
But VACUUM advance moves the trigger (or points) in rotational degrees "or time" and changes the rotor phasing. The rotor alignment must have enough overlap in alignment so that the change with vacuum does not get the rotor "far off" the contact. In a perfect world, the rotor would be exactly centered when the vacuum advance is at the midpoint of it's movement
Of course most of us do not have "adjustable rotors" but this still illustrates
