The interline, also known as interline and interline ticketing, is a voluntary commercial agreement between different airlines to treat passengers travelling on routes requiring several flights with several airlines.  Such agreements allow passengers to switch from one flight with one airline to another airline flight without having to pick up their baggage or re-register it. Airlines can also promise a free booking change if the service is interrupted due to a delay. I think Air India is very unreasonable. Indeed, it is clearly written on their website that students travelling on a student visa can take „Free 1 additional piece to Europe interior points via Frankfurt/Paris/Rome/Milan , subject to an interline on the rates published non-stop on the itinerary) (maximum weight 23kg). If there is no interline ticketing agreement, two separate tickets must be issued and passengers must pick up their luggage and take it to the connecting company for check-in. Interline routes such as this one are more risky for travellers, as the second airline may not be aware of inbound flight delays or problems and is less likely to authorize a toll-free change of booking in the event of a loss of the route. There may also be a problem if the baggage is lost and the traveller wishes to be sent to them later. Interline agreements are the most basic types of agreements you can have between airlines. An Interline agreement is simply a commercial agreement between airlines to treat passengers when travelling with several airlines on the same route. This allows passengers to check their luggage to their final destination, check to their destination, possibly be re-routed to another airline in case of irregular operation, etc.
In short, let`s go back to these four types of agreements: If a codeshare contract is like dating, then a joint venture is like getting married. A joint venture agreement is a massive business decision that usually requires full government approval. When airlines create a joint venture, they coordinate prices and schedules and have an agreement on revenue sharing. In all of these partnerships, the exact details will vary depending on the agreement reached. I hope this will provide a useful overview to demystify some of the partnership terminology, often used by airlines around the world.