I am not entirely sure about all of this, but I will attempt to explain things as I understand them. Someone please correct my mistakes. Prior to departure there is the waitlist for upgrades and a waitlist for awards. For an upgrade, one is awaiting NC/NF space to open up. For awards, one needs XY/XC/XF space to open up. According to what I have read here, there is a PA, PB, and PD waitlist. While I am not positive, I think the way it works is that the entire PA list is cleared based on dated added, and then the PB list, and then the PD list.

At a certain point prior to departure, the airport takes control of everything that hasn't cleared yet. At this point I believe everyone awaiting an upgrade gets up on the UGDG list, which I believe is part of the Departure Management List. This list is cleared based on status, fare paid, time added, and then alphabetically based on last name. However, someone who has a BP code of FFCC (meaning paid for F/C, but isn't confirmed on a flight) on a flight will trump a 1K who is trying to upgrade. Some FTers have been successful getting FFCC on awards even when s/he was never confirmed in an F cabin, but did get an F award. For example, if one has a XC award for JFK-LAX-SEA-LAX-JFK, and everything has cleared except LAX-SEA, some FTers have gotten CSRs to make them FFCC on the DM so that s/he clears before 1K just trying to upgrade using miles/certs.

There is also the standby for passengers who desire to take a flight other than the one on which s/he is confirmed. Once a CSR beings to dump confirmed reservations and passengers, the CSR will then start to clear the DM list starting with BP5As. BP5A is generally reserved for disserviced and full-fare passengers. However, some FTers have been able to get BP5A even when on cheap fares. The BP5A list is sorted by status, fare, time added, and then alphabetically by last name. After the BP5A list is cleared, if seats are still open, the BP5B list is then cleared, which contains all other revenue standbys. It is cleared in same order as BP5A. This means that a 2P with a BP code of 5A will receive a seat before a 1K with a BP code of 5B. I think the BP5A code is rarely used. If seats are still open after all revenue standbys are accommodated, non-revenue passengers (BP5C through BP10) are then accommodated on the flight. These are the passengers referred to as NRSA. There is also a PSUG list for UA employees traveling on company business who are allowed to upgrade to the next class of service. This list is cleared after all revenue upgrades are accomidated. However, I am not sure exactly how that works, or how NRSAs travel in premium cabins of seats are still available.

I hope have gotten all the information correct. If anything is wrong, please correct me, and I will change it, but this is my general understanding based on FT, my experiences, and conversations with CSRs/RSSRs.