https://www.united.com/en/us/fly/mileageplus/benefits/mileageplus-air-awards.htmlWaitlisting for travel awards is permitted in premium cabins on flights operated by United, and will be automatically confirmed, if the waitlisted cabin is available, based on the status of the account holder.
Waitlisting may be requested for travel in a premium cabin up to 24 hours before flight departure. If the waitlist does not clear, you will automatically be added to the airport standby list upon check-in.
Waitlisting for certain inventory types is not permitted.
Ticketed customers may waitlist for travel in a premium cabin for an alternate flight. Reservations where all segments are waitlisted without any confirmed segments are not permitted.
Connecting flights: Customers may waitlist on only one leg of the connecting flights and the flight must meet the required minimum connecting time.
Nonstop flights: Customers can waitlist on only one flight per direction. Alternate dates and times are allowed.
You may use miles for travel in United First®, United Business® or United Polaris® business class, even if the space is not available. In these cases, United Economy in the same award type must be confirmed, and the front cabin will be waitlisted. If the courtesy waitlist does not clear, it will expire 24 hours before itinerary departure, and you will automatically be added to the airport upgrade standby list upon check-in. In these cases, you will be confirmed on a space-available basis by a United airport representative at the gate once the flight has closed for check-in. For United Polaris business class and United Business travel awards, the difference in miles will be refunded when the United Polaris business class or United Business class segment of the trip cannot be confirmed. Waitlist requests may only be made over the phone with your local United Customer Contact Center.