Originally Posted by
BenA
Actually not for the baggage, if you are on one ticket; if your HA segment is on the same ticket as the longer flights, DOT rules for baggage apply - meaning the most significant carrier's rules are applied. For an international trip, the MSC is the carrier operating the first TATL/TPAC segment. For domestic, it's the first carrier you fly on the entire ticket. Those rules are applied through the entire trip, outbound and return, since the DOT requires them not to change.
You oversimplify the DOT requirement. On international tickets the IATA Most Significant Carrier rule is one of two mechanisms that can apply. Domestic doesn't use MSC at all, and it depends whether the first segment is a codeshare or not.
If the OP didn't even look at cabin of service or seat assignments he doesn't sound like the type to get into DOT baggage allowance arcana of 14 CFR 399.85.
For those who do want a deep read, see page 27 for the start of a 16-page FAQ.
https://www.transportation.gov/sites...TE%20FAQ_2.pdf