Non Revs are standing by for the flight in either F or Y depending on how they list themselves. The system is automated when they clear the list. If there are no remaining F seats they will be cleared in Y based on their pass classification and seniority. You can stby for F if you purchased a F ticket, if you were upgraded from a Y ticket, you stby for coach, then you can be upgraded after that if seats are available. By the way, non revs cannot be cleared until all rev stbys are cleared first.