Originally Posted by
findark
As the itinerary touches the United States, the DOT rule preempts IATA convention and the marketing carrier of the first segment ("first marketing carrier" - UA) determines the allowance of the entire ticket. What's not clear is what class of service should apply.
MSC applies to journeys which do not touch the USA.
Although, in a frustratingly confusing caveat, UA is permitted by the DOT to defer to the MSC anyway. If they choose to do that, it has to apply throughout the entire journey.
What's on the receipt?