Originally Posted by xliioper

Upgrade priority is based on the highest level person in the reservation. It doesn't matter whose account the miles came from.

Note that while most international awards from US require a roundtrip booking for lowest award levels. Domestically, it varies depending on how competitive the route is and if there are roundtrip revenue fares on route. If you are flying out of SEA domestically, DL competes with AS (who only has one way fares) and most DL revenue fares (and also awards) are only one ways to match AS. Out of captive hubs like DTW and MSP, there tend to be more domestic routes that require roundtrip bookings of revenue/awards to get the cheapest fare/award levels.