To Unassign External Devices, you need to select Unassign from the mouse right-click menu on the assigned control knob.
But this can only be done one at a time.
When there are many assigns, it is difficult to perform Unassign one by one.
How about installing an [Unassign (all)] button?
It would be very nice if Unassign could be executed for all assigns in one operation.