The default controller for motor on/off is "A~(w)".
It's definitely not Dim A. What is it?
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A0 is motor off.
B0 is motor on.
On an 88 note keyboard, those are the two left-most white keys.
The "(W)" means "white." VSL uses this to designate that the switches are only on white keys (they're not chromatic).
In the UI on the same banner as "Synchron Player" and to the right, you'll see a keyboard icon. Select that and a keyboard graphic will appear that specifically labels each keyswitch.