Datum - Datum reference (A-B) simply means that one is creating a single datum reference from multiple separate features. If datum A and B where two separate plane surfaces - then the datum element would be a single datum plane derived from the true geometric counter part of both datum surfaces simultaneously.
I'm not sure what the intent is for datum A = plane and datum B = cylinder in this application and would need to think about what the end datum reference plane(s) and axis might look like after simulation.
My initial thought is that this is a drawing error. Can you post an image of the engineering drawing where this specification is show?