With mixed experience on UA gate agents' willingness to process standby for earlier flights with checked baggage, I wonder about UA's official policy. I really don't want to arrive at the airport several hours early if unable to standby due to the checked baggage issue.

Obviously, if my checked baggage doesn't make the earlier flight, I understand that I will need to pick up from the airport (no UA baggage delivery service provided), but when checking baggage several hours before departure, I've found that UA often loads the baggage on the earlier flight anyhow.

Can someone shed light on official policy?