Thank you very much for the input.

The certificate is certainly eligible for international travel. Since it seems to be usable onlyfor online booking, I never thought about calling the customer support. I appreciate the help.

If anybody cares to know how I got it: I was on UA 940 ORD - FRA in business (using SWU) and my TV didn't play movies. So I had the option of getting 10,000 miles, -20% on economy international ticket, or $250 domestic roundtrip voucher.
Considering I was flying east and wanted to just watch 1 movie, it actually wasn't a big deal to just sleep and not watch TV, but it was a welcomed United perk.

Thanks for the help!