Originally Posted by Andriyko

All flights would be on one record locator (all AA flights) but there won't be a corresponding BA locator per se. BA will create an operational PNR for its operated flight. You don't normally get the operational PNR number (you need to call the operating carrier and ask them to find you on the flight if they have access because many code-share agreements only require submitting the passenger list 24 hours before the flight). It works differently when you book a prime flight and a code-share flight.

For example, when you book LAX-JFK(AA)-JFK-LHR(AA*BA)-LHR-FCO(BA) on BA stock. The original BA PNR will have all 3 flights, then there will be an AA PNR for its prime LAX-JFK flight and then BA (as an independent process) will create an operational PNR for the JFK-LHR flight operated by it. BA can't create an operational PNR for its flight in the original PNR as it will look like this:

LAX-JFK(AA)
JFK-LHR(AA*BA)
JFK-LHR (BA)
LHR-FCO(BA)

But the real reason is that BA does not even know that it is a code-share when the booking is made. The operational PNR is created independently. And in instances when you have prime BA flights and AA code-share on BA flight in the same itinerary there will always be a separate PNR for the code-share flight (not only that, but each code-share flight will be in a separate operational PNR for the operating carrier - this is how these flights are managed).