I am trying to understand what form of "discrimination" OP's father experienced.

Even if UA's internal policy made him #1 and he received #24 , that alone does not make his treatment "discrimination".

If this is all done correctly, FA's no longer flit around the cabin taking orders. Rather, they take the orders in order and then sort out who gets what if not all orders and then return to affected passengers, if any, for a second choice.