Geordie405

I'm hoping someone can shed a bit of light on why we sometimes end up with multiple PNRs for a single booking. Two examples to get the ball rolling, both of which were booked with AA:

1. LAS - DFW - LHR - NCL - LHR - LAS where I have one AA record locator, a BA PNR that lists DFW - LHR (operated by AA) and all the other sectors (all operated by BA), and another BA PNR that just lists NCL - LHR - LAS.

2. NCL - LHR - LAS - LAX - LHR NCL: where I originally had one AA record locator and a single BA PNR covering all flights, but since I upfared from PE to Business I now have two BA PNRs, a new one listing NCL - LHR - LAS, and the original still listing NCL - LHR - LAS - LAX - LHR - NCL but where I can't select seats for NCL - LHR and where the BA app shows no ticket number for this sector.

Clearly it's possible, in spite of the different systems used by AA and BA, to generate a single BA PNR to match what's on the AA record locator. Scenario 2 above illustrates that. However, when I upgraded and the ticket was exchanged / re-issued by AA we ended up with two PNRs. What caused the 2nd PNR to be generated? Equally, for scenario 1 why did we end up with two PNRs from the outset?

Is anyone sufficiently familiar with the IT behind these systems to explain from a technical perspective what drives the PNR generation and how it's determined which flights make it onto a single PNR and / or what causes flights to be split across multiple PNRs?

Thanks!