UAL's off line policy is that the number of JSers allowed is equal to the number of JSeats installed in the a/c. IN ADDITION there has to be an open seat in the back for you to sit in. Unfortunately, you also have to battle the gate agents who for the most part think they OWN the jumpseat,and that THEY will dicatate wether or not you get it.
Priority is mainline, express, and then others. If you fall within the categories, it will be seniority based for mainline, and first come first served for the others (Express included)
It's a shame in this day and age that they do not have an unlimited JS policy. (Actually they do- in they're last contract they had/have a clause which states that in case the company starts to charge for pass writing again- which they have done- then the company will allow as many jumpseaters as there are open seats. HOWEVER- since the whole CH11 debacle, spending in excess of $300k to reprogram their load planning software and reservation systems has fallen by the wayside. All of this explained to me by a ticked off UA ALPA guy who was intent on filing as many grievances as he could since they started bending the pilots over.)
Enjoy the Un-friendly skies.....