Originally Posted by
Often1
Under US DOT rules, if the ticket touches the USA, which this one does, the allowance for the first segment becomes the minimum allowance for the entire itinerary. What is unclear to me from your post is whether it is AA or BA which is the marketing carrier and it is the marketing carrier's rules which will apply.
That is correct.
The U.S. regulations do not require airlines to follow the "most significant carrier" rule. The IATA does, however. Under that rule, AA is the MSC because of the LHR-DFW flight. Stopovers make zero difference, I.e., are wholly irrelevant.