The record locator is an identifier that is unique within each reservation or GDS system. When you book a PNR in an airline system, such as via Delta.com, a DL record locator is created to uniquely identify your record IN THE DELTA RES SYSTEM. If you had a NW segment in the DL PNR, the DL system would send a subset of all your data to NW and NW would create a PNR with a NW record locator within their system.

If you book through the usual internet sites, it gets even hairier. The site (e.g. Expedia) interfaces with a GDS (e.g. Worldspan). If you booked a trip with a DL segment and a DL codeshare on CO segment, there would be 1) a record locator specific for Worldspan, 2) a different record locator in DL, 3) another (different) record locator in CO , and you also have all this itinerary data stored by Expedia in their own itinerary database.

Bottom line... if you want to be able to display your record by record locator, you have to know what system the record locator came from and get access to that system. The various sites mentioned above likely interact with only one system, so in most cases, if you just randomly try a record locator... it won't work.

Sorry for the lengthy description...