Originally Posted by
angra
Hopefully someone can clarify soon. The rules as described in the link I gave seem to imply that as of 2018 it no longer matters, but I'd hardly put it past Delta to have a contradictory rule buried somewhere, or to just flat out not properly state the rules.
It doesn't matter. Just go by the flight number. If it's DLxxxx it earns as a DL flight. If it's MUxxxx it earns as MU, no matter who issues the ticket.