Originally Posted by rcfd13

Your domicile and seniority determine what you fly. At Skywest PHX based crews fly all US Airways. SEA and PDX RJ crews fly Alaska. LA crews do all of the American flying. If you're not based in those places you won't touch an Alaska, AA or US Airways RJ unless you get a TDY there.

In LA the AA flying tends to go junior so the more senior people will bid the Delta and United flying. Delta trips tend to go more senior in any domicile because it's mostly 700 and 900 flying which pays more.

There's nothing limiting what carrier a person can fly for, but based on seniority and domicile you can make a pretty good guess about what carrier a person will mostly be flying for.