Originally Posted by emcampbe

Can only use it to purchase flights. Theoretically, no T&Cs specifying UA/UAX only, but the option won't be available if you have a codeshare or OAL flight on any segment.

Similar to using a TVC - during the purchase process, you select "United Payment Options". Then click the area to select Travelbank, and then you will be presented with a slider allowing you to select the amount you'd like to use (up to the available amount, of course).