I was handed a meal voucher at IND. I was not getting a clear answer from the agents so thought of asking the experts here.

1. How do I know which merchants are participating? I couldn't find any list/ hints anywhere on united.com.
2. Do I have to use the voucher at the airports? If yes, does it have to be at the airport where the voucher was issued (IND) or can it be at any airport anywhere?

I apologize if this has been answered before. Couldn't find anything.