Primary cardmembers can use their miles to book any available seat, any time on any United - operated flight when they redeem miles at the MileagePlus Standard Award level. (uponarriving.com)
As an example of how one program works, United normally offers its saver award seats in the lower 48 states at 12,500 miles each way, but when a saver award isn't available, those with a United co-branded credit card or United elite status typically can secure an award seat at the 25,000 - mile standard award level each direction, as long as seats are still available for sale with cash. (creditcards.com)
If seats are available for sale, they can be redeemed at the MileagePlus Standard Award level (uponarriving.com)