This is a United "Travel certificate" and not "future flight credit"
Check the terms here: https://www.united.com/en/us/fly/travel/credit.html
How to use a travel certificate
- Retrieve your certificate details including the year issued, PIN number and last name. This information is included in the original email you received when the certificate was issued; to have that email re-sent to you, use the tool above. You can also find these details in My Account.
- Apply the certificate on the payment page when booking a new flight.
You can use travel certificates to buy tickets for flights operated by United or United Express. You can also apply travel certificates toward some non-ticket items, like Economy Plus® seating, Basic Economy seat assignments and preferred seating when booking your ticket. PlusPoints upgrades can't be combined with travel certificates when you're booking a flight.
Once your travel certificate is issued, we'll send you an email with all the information you need to use it, including the year of issue, PIN code, and last name.
If you’re a MileagePlus member, you can view any travel certificates you’ve received by visiting the My Account page.
If you don't have an account or aren't signed in, you can have the email with your certificate details re-sent to you using the tool above. You can also use it to look up a travel certificate if you don't know the name, year or PIN number.
Can I combine travel certificates together?
Yes, you can apply up to ten travel certificates in a single transaction when booking your ticket on United.com or United mobile apps.