I would consider it a supplemental since it's not a regularly scheduled flight for Airline A.
But perhaps Airline B could wet lease a plane and crew from Airline A to fly under Airline B's certificate, then that might be considered domestic... but I don't know much about that other than cargo companies do it all the time... (and this might be one of the deviations a company can use, so it may not count)
I have seen some city's that we normally don't fly to, but they have the normal flight numbers. We usually fly there for like a week (maybe) and then I don't see them again for a while. Perhaps if it was known far enough in the future it could be considered a scheduled flight?
I contend it can be considered a scheduled op still. From the DOT : "Scheduled Service: Transport service operated persuant to published flight schedules, including extra sections and related nonrevenue flights". I think the extra sections can cover the flying for a regional carrier that. Also, this statement "Scheduled Service: Transport service operated over an air carrier's routes, based on published flight schedules, including extra sections." There's another statement (i lost it) that refers to individually ticketed passengers or individually waybilled freight. Since the pax are individually ticketed, and not paid for as a group, I think that lends credence to op-ing as a scheduled flag/domestic. (In a separate context there's a "public charter" with individual ticketed passengers operated by a supplemental carrier but there's some extra rules that go with that your standard carrier doesn't deal with)
We fly a lot of extra sections as volume demands. So, you can argue that this flight is just an extra section added for volume to the airport. You don't know why you need to operate a bigger plane (or extra flight). We have had incidence where we, as Airline A, had contracted to a carrier (Airline B) to fly a route for us. Airline B was a supplemental carrier, and had a broken airplane. Airline A used a spare to op the leg. Still considered a domestic/flag extra section.
Well from the dispatch side one would never see the contract or even would know what is in it.
All the information you get is that you have an additional flight on your desk, that is not a normal mainline flight number.
Also would it make any difference if the city pair were listed in C70 as regular?
We just went through a large schpiel about how a flight that's diverted to an approved alternate can be dispatched from said alternate (if it's not a regular, provisional, or refueling airport) to a regular airport under domestic/flag rules. I don't remember all the justification used but the FAA bought it.
Question, when your carrier operates charters, say for sports teams, between C70 airports; are they operated as Supplemental? We get briefs from out charter dept for such flights and still (under A030) operate them as a standard flag/domestic. I'm guessing that as a large enough carrier to have a regional operating for you that there's a charter department that sends out info for such things.
OK, that's my opinion, could be wrong. It was a long overnight shift followed by a couple of adult beverages.