This info can be very hard to find, I had a hard time anyways, never really did find it from my manufacturer.
My cove was notched at the base to account for the bottom rail, my logic was: if I put sand first than what is the point of the notch? It's just empty air because the pool base will be even with that bottom rail.
I wanted the notch to be reinforced by the rail, so I put the cove down and then the sand - being careful where the 2 meet.
If there is no notch in the cove I'd put on top of sand.
I'm not saying what's right from wrong, but 2 summers in and my cove is fine.