Originally Posted by
oliver2002
What UA and AC do is not add OSI notes, but insert ghost/passive segments into the booking, adding the PNR of of the other booking in the passive segment. So the UA agent checking you in will see that you are continuing on the other airline and the luggage tag prints that way. The otehr airline see you are connecting from another flight.
I saw that in action when I asked to check thru luggage onto the separate LH flight from ORD.
I don't think that's what OP is talking about. My understanding of "linked PNRs" (a very North American idea IME) is that pax 1 is on PNR AAAAAA and pax 2 is on PNR BBBBBB and they are both traveling on the same flights. UA or AC will offer to "link the bookings" for IRROPS/seat change issue/etc.
Ghost/Passive segments can only be used when it is the same pax flying on multiple tickets, and the carrier wants information about the inbound/outbound connections.