Originally Posted by jsloan

Yes, there would.

UA's schedule would be absolutely impossible to manage if they used O/A/R as domestic first codes, and then magically threw in J/C/D/Z/P whenever they happened to be using a three-class plane.

UA defines its service commitments by market. If you fly IAH-EWR, they don't guarantee a lie-flat seat. If you fly IAH-LHR, they do. It's spelled out quite clearly during the purchase process.