"1K Priority Reservations
You have priority status to acquire seats on United flights, even on many sold out flights, on all United (excluding TedSM)and United Express® flights anywhere. However for all priority reservations for you, and one traveling companion, you must call 1K Reservations no later than 6 hours before your flight and no earlier than thirty* days in advance, and purchase an E-TicketSM in United Economy® Y booking class fares**."

http://www.united.com/page/article/0,6722,1171,00.html