It is for everything above 500 in the Align fleet any way.
450>325
500>425
600>600
700>700
It used to be judge by motor power, but every factory has there own system now.
If I remember back at one time the size was based on the motor. Now it's the blade length that determines the size. I think those blade sizes are standard sizes, my 450 uses 325mm blades or I can opt for a 350mm blade (stretch kit)