What happened when the flight is operated by another that the one bought ?
When the flight is operated by another company that the one the passenger bought his ticket with, this is commonly referred to as a “code-share agreement”. Which is an arrangement where two or more airlines share the same flight. So, a ticket can be purchased from an airline on a flight that is actually operated by another airline under a different flight number.
Code-share agreements are an important detail if you are flying into the EU and it comes to eligibility.