IIRC UA P mapped long time to AC Z.

AC P seems to have been a low economy (tango) booking class, which was changed a few monts ago... Now it figures as business booking class, but not available on the whole network (mostly international routes outside North America)...

You might check this threads to get more information or search by google:

False P Fare Strikes Again (UA codeshare on AC metal)

Fare Code Mapping for UA Codeshares on AC Metal