Originally Posted by
serfty
I agree, but this needs to be clear as it can have significant ramifications with IATA 302.
If all travel in on AB metal and none to/from the USA, it's AB baggage allowance that matters - nothing else (presuming the OP is not Emerald).
This is what applies to your flight. So it's the Marketing carrier's rules that apply. (AA & BA in this case)
U.S. Exception (US DOT Order 2009-9-20 <05Oct09>)(a) For passengers whose ultimate ticketed origin or destination is a U.S. point, the baggage provisions selected at the beginning of the itinerary shall apply throughout the itinerary, regardless of stopovers, and
(b) in the case of code-share flights that include a point in the United States, the Most Significant Carrier (MSC) shall be the Marketing Carrier.