Is the first carrier on the itinerary the first flight on the entire roundtrip or the first flown on a leg?
For example a trip LHR-SEA on BA with a return leg of SEA-YVR-LHR, with the SEA-YVR on AC, but YVR-LHR on BA. Would the allowances for BA or AC be used for the return. Would an allowance printed on the ticket overrule the general allowance from AC?