Originally Posted by jsloan

P books into the next cabin above economy. On most domestic flights, UA sells First and Economy. On international flights, UA sells Business and Economy on some planes and First, Business, and Economy on other planes.

C, D, J, and Z all behave the same way.

Note that UA's domestic first class seat is normally a few inches wider than economy with a few extra inches of legroom. Don't expect the kind of experience you'd get on a long-haul international flight.