Per United Website under saver award rules:
Waitlisting for travel awards is permitted on flights operated by United, and will be automatically confirmed, if the waitlisted cabin is available, based on the status of the account holder. Courtesy waitlists may be requested and held up to 24 hours before itinerary departure. Confirming a Standard Award and waitlisting for a Saver Award is not permitted.
BUT... as I have read and experienced... being put on a waitlist (that even showed on the website itinerary) does not mean it is automatically confirmed. I was waitlisted for a flight, saw it open, saw I was still "waitlist", called UA and the agent confirmed me in 5 min or less.