In the world of travelling, PNR is a way to enroll in the database of CRS (computer reservation system) which comprises the itinerary for one passenger or a group of passengers that are travelling together. The necessity of PNR was firstly realized when passengers found difficulty in getting information about specific flights from multiple airlines to move to their desired place (interlining). To make it easier, IATA and ATA have introduced interline messaging of PNR along with other data with the help of "ATA /IATA Reservations Interline Message Procedures - Passengers" (AIRIMP). For PNR status check, no blueprint or layout has been yet defined.
CRS or hosting system has set its proprietary standards catering to the general industry needs which include the requirement to track PNR data easily for AIRIMP messages. This has eventually led to assorted guidelines in data format and content amid different major systems.
When the passenger informs the travel agent/website or personally books an itinerary, it generates a PNR in the CRS or system, which it uses. This is acknowledged as one of the largest global distribution system like Galileo, Worldspan, Sabre and Amadeus. However if the booking is done personally with any airline, then the PNR will be in the database of that airline's CRS, which is also commonly known as the MASTER PNR for that specific passenger and related itinerary. The record locater identifies the PNR in the particular database.
In case, if the keeper of master PNR does not rendering portions of the travel, then copies of this PNR information are delegated to the CRSes of that specific airlines which will be providing transportation. Following this, the CRSes shall open the copies of actual PNR in their respective database, and try to manage the portion of itinerary for which they are accountable.
Most of the airlines hire their own CRS, which are hosted by the GDSes, which gives the freedom to share the PNR.