Wikipedia has a short entry on booking references, but does not give any detail on how they are generated. Each booking system, whether travel agent, airline or bus company, creates a booking reference (BR) as a key to the Passenger Information Record (PIR) in its own database when it records the booking. If a booking involves multiple carriers, and they use the same service, such as Amadeus or Sabre, then there is a single BR. If one uses Amadeus and the other Sabre (or some other system), then the passenger gets to see the original BR, and the systems negotiate behind the scenes so that the second system recognises the booking by its own BR, but can use other information associated with the booking (or its bit of the booking) to identify what it is responsible for providing.