Air Ticketing.

Since the modern concept of air ticketing was first coined by American Airlines in 1952, it has played a key role in the flight booking industry. And while the advent of the e-ticket has relegated manual issuance and paper tickets to the history books, the importance of ticketing is the same as ever and understanding its role is vital for any travel industry newcomer.
Tickets entitle passengers to a seat on the flight - for travelers, a ticket is a confirmation that the seat has been paid for and that it’s theirs – no one else can take it. In this sense, it’s a travel document. For the airline, the ticket contains information about the booking like the PNR number, passenger details and information about the itinerary, fare and payment.
Tickets create a contract between the passenger and the seller - whether its bought directly from an airline or through an agent, a ticket is what seals the contract and governs the conditions and responsibility for post-booking services like changes, cancellations or refunds.
Tickets establish responsibilities when multiple airlines are involved - some itineraries involve multiple airlines working under a codeshare or interlining agreement. In these cases, a ticket helps divide responsibilities between the validating carrier (the airline that sold the ticket) and the operating carrier (the airline operating the flight).