When you are hired, they tell you what certificate and airplane you are on. You ONLY fly for that certificate (although you may fly multiple codeshares i.e. Delta Connection and US Airways Express), and you only fly that airplane. You can, however, change bases whenever your seniority can hold the base you want. You are "locked" in the certificate and airplane until you can become a captain. Once you can hold a captain slot, you can transfer to another airplane or base depending upon the opening you are taking. (all certificates on a master seniority list) As for the bases, my class did not find out our domiciles until a week after the checkride. And the initial assignments generally ignore what you request. Your first base is going to be where THEY need you. For example, the majority of the last several classes to finish for the CHQ 145 got Buffalo... hope this helps