If you are using a SWU/CR-1, then you are on the PA waitlist.

If you are using a mileage upgrade award, then you are on the PB waitlist.

UA clears the PA waitlist first, then the PB waitlist. These waitlists are cleared in "order of addition", so the first person added clears, then the second, then the third, etc. as UA Inventory Management make upgrade inventory available (the "X" and "O" buckets).

If you have not cleared before roughly 24 hours, the waitlists then go to Airport Control. Both the PA and the PB waitlists are dumped into a master Departure Management (DM) waitlist. This waitlist is then cleared first by status (in order, UGS, 1K, 1P, 2P, GM) and then by fare paid (C, D, Y, B, M, H, etc.) and finally, if necessary, by time added to the DM (you are added to the DM when you check-in for your flight).