Originally Posted by
jackal
U does not equal A, although in practice, I thought I've seen that usually A>U.
A is always a subset of U, which in turn is always a subset of F.
If there are 3 F seats for sale, there can be 0, 1, 2, or 3 U seats available.
If there are 2 U seats available, there can be 0, 1, or 2 A seats available.
A space can open up at any time, but the closer you get to departure time, the more likely it is there is an AS or DL elite who has purchased a ticket and waitlisted for the U space.
It's not entirely clear who is first in line for an A seat when there are both premium award ticket-holders waitlisted for A as well as elites waitlisted for U.