Interlining clarification

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Once you have Interlining agreement with a holding Company, do you have to open a station on the contractual partner airport? ex. My holding is in the Philippines, wish to IL with Japan Holding, I have these MNL-NRT routes and the partner holding has domestic routes, do I have to open station in Nagasaki for my MNL - NRT - NGS to work efficiently?

No, you don't need to open additional stations for an IL to work.

But you have to sign interlining switch the company that actually flies to your airport. If it’s a pure holding (without flights), then the IL is useless.

If there’s one company with two subs, you will have to IL with both subs.