Originally Posted by jsloan

You're mistaken about P. P is a cash inventory bucket available to everyone, not an upgrade bucket. PN is upgrade space for GS.

P9 PN9 indicates (a) there aren't likely any GS members waitlisted, and (b) UA doesn't think they can sell all of the seats. A flight that is P9 PN9 may open PZ space next if the sales numbers continue to lag behind UA's hopes. In general, if you have to waitlist an upgrade, all other things being equal, a flight with a higher PN value is more likely to have PZ open than a flight with a lower (or 0) PN value.