American sells tickets up to the average no shows for a particular flight. Sometimes everyone shows and that's when the flight becomes oversold. Regardless, you'll see offers on the app. They do that to avoid bidding wars at the gate and potentially saves them some cash. So regardless if you voluntareee or not, you will know if is accepted or not at the gate. The only exception to this is if an IROP affects a CK or an urgent pilot deadhead takes a seat. In that case the volunteers offer is done at the gate.