I was more speaking of international.
Prices where the same marketing carrier is designated on all sectors is an 'on-line' price and carriage is on-line. It is the marketing carrier whose price and terms and conditions apply. At least that is what regulators say.
Take GVA-YVR. BA/LH/KL will all have on-line prices on flights using their own metal the entire journey. AC will have on-line prices where LH is used GVA-FRA and/or FRA-YVR and their code is on one or both flights. Funnily enough AC does not code-share with LX on LHR-GVA, so a request for that itinerary gets you sum-of-sectors (interline) as does BA-LHR-AC.