While anyone can be bumped the procedure that Southwest follows is that they bump those who don't have a boarding pass. This is done by when you check in on line, or at the airport, instead of getting a "C" Boarding Pass you will get a Security Document that states "Proceed to Gate". At the gate they will either give you a "C" boarding pass or note the time that you arrived at the gate. Ten minutes before the flight they will issue Boarding passes to those people, in the order that they arrived at the gate, to replace the volunteers who are not going or passengers who just don't show. For those who have an "A" or "B" Boarding pass they will only get bumped if they volunteer.