I worked at UA Reservations from 1985-87.

• PA stands for "Priority A" Waitlist
• PB stands for "Priority B" Waitlist
• PD stands for "Priority D" Waitlist

I no longer recall if there was a PC that was used very rarely if at all, perhaps for company (UA) positive space?

These abreviations are common throughout the industry, like "HK", "WL," "NN", etc., even though the exact way the airlines clear their waitlists may differ.

Back when I worked at SFORR, a PA was only valid within 24 hours of travel. So getting waitlisted as a PA further out did not help you. The Reservations Sales Reps would list people as a PD, or PB if Premier or had other urgent reasons for travel.

PA was only done within 24 hours of the flight departure; you would be dis-serving the customer by listing him earlier. Occasionally, a knowledgeable elite passenger would ask to be listed PA, and I would explain the 24-hour rule, suggesting he call back then to be changed from PB to PA.

Maybe only the PA queue is cleared within 24 hours, except perhaps at the airport, but I never worked on queues or at the airport, so at this point, 18 years later, I'm just guessing on that. I do know that PA was higher than PB within that 24 hour window.

This may have changed in the past 18 years, but I don't think it is some big, deep, dark UA company secret.