Originally Posted by
Jenzo
Take the BA booking reference and enter into Cathay MMB.
This will not work as the BA booking reference (even though CX also uses Amadeus) contains the "marketing segment".
The "operating segment" is housed in another CX PNR [the "operating PNR"] (also on Amadeus) which you will need to obtain by calling CX and them the equivalent CX flight number, date of travel and passenger name.
Seat assignments can only be done in a PNR that has the actual "operating segment (CX XXX)" and can't be done in a PNR with the "marketing segment (BA XXXX)".