The marketing carrier of the 1st segment chooses whether to apply its allowances or defer to that of the MSC
The other key thing is that the allowance applicable at 1st check in applies to all check ins
Cathay's site has a nice description
http://www.cathaypacific.com/cx/en_A...r-journey.html
Originally Posted by Cathay
1.For journeys to/from the US, the rules of the first marketing carrier on the journey apply, provided that the first marketing carrier is specified on ATPCO’s (Airline Tariff Publishing Company) published list of US DOT Carriers (defined as carriers where general rules tariffs to/from the US have been filed).
2.If the first marketing carrier is not specified on ATPCO’s published list of US DOT Carriers, then the rules of the very next marketing carrier on the journey that is specified on ATPCO’s list of US DOT Carriers apply.
3.Once the first marketing carrier is identified (through steps 1 and 2 above), it is the first marketing carrier’s policy to opt for the rules of the Most Significant marketing Carrier (MSC) for the journey or retain the first marketing carrier’s own baggage policy. (Cathay Pacific opts for the rules of the MSC.)