You can (voluntarily) standby with checked baggage for international flights to Canada and the Caribbean. A yellow STANDBY tag is attached to the baggage and the bags are not loaded unless the passenger clears the standby list. You can purchase same-day standby online or at a self-service machine if offered, qualify for same-day standby through elite status, or standby for the next flight out if you miss your original flight.

The STANDBY tag should technically be attached to domestic baggage as well, but it doesn't really matter since there's no need for positive passenger bag match and the bag will usually go out on the earlier flight regardless.