Why you’re asked for it
Airlines can be fined if they carry someone who is later refused entry, so at check-in they may ask for proof you intend to leave. Immigration officers ask for the same reason. You need something real and checkable.
The methods, compared
There are four common approaches. They differ sharply in cost and reliability.
How to make it bulletproof
Whatever you choose, confirm the PNR resolves on the airline’s own website before you travel. If it doesn’t show your name, route and dates, it won’t survive a check.