"Seating assignments" refers to the process of determining where someone will sit or be assigned to sit in a specific location or event. Full definition
You'll be charged for advance seat assignment or meal service for the roughly six - hour flight. (dannydealguru.com)
Though the restrictions aren't complete — no mention of the ability to pay for seat assignments at 48 hours for example — you can't honestly say you weren't warned. (travelcodex.com)
Coming down the pipeline, look for higher fees for award tickets — whether booking fees, seat assignment fees, or change fees — and a less generous award chart. (rewardexpert.com)