What I have seen is that if, at the time the convention is called to order, there are vacant delegate seats -- whether a delegate did not appear and is not registered, or is registered but is not present on the floor for whatever reason, an alternate will be seated. If the delegate decides to make an appearance later in the day (or next day!), the alternate who is asked to relinquish his seat can feel abused. Yes, there can be abuse on either side. I don't know what the rule should be, but it should be very clear. You can lose a lot of floor time deciding who has the greater right to that seat.