Originally Posted by
3Cforme
There is an international upgrade waitlist. People using miles to upgrade may go on it. People using GUCs go on it and clear into available inventory in advance, or into empty seats at the gate.
Then seats go to non-revs.
If not filled with non-revs, seats in D1 can fly empty.
There is no mileage upgrade wait list if not cleared 24 hours prior to departure.