What are codeshare and interline flights?
A codeshare is a flight operated by one airline but sold under another's flight number; interlining is an agreement that lets one ticket cover flights on several airlines with through check-in.
In a codeshare, the marketing carrier sells the flight under its code and the operating carrier flies it. Passengers may see two flight numbers for the same aircraft. Interline agreements allow itineraries combining different airlines on a single ticket, with baggage transferred between them.
Engineering implications
Booking systems must track marketing and operating carriers separately. Baggage rules, check-in, seat selection and disruption handling can depend on either. Displaying the operating carrier clearly is also a regulatory requirement in many markets. Record locators may differ per carrier on the same itinerary.