Originally Posted by
Zwiebelbauer
Based on IATA rules, DL's rules don't necessarily govern -- they just get to decide whose policies apply.
In this case, I think under standard MSC rules, whichever airline is marketing the LAX-MEX leg (regardless of who it's operated by) would be the most significant carrier as that'd be the first international sector. However, standard MSC rules don't apply here due to the USA rule that always defers to the marketing carrier for the first leg.
Your post is confusing. IATA rule do NOT apply when origin/destination is in US (or Canada). US DOT rules apply (Canadian Transport for Canada)
While US rules allow carrier to pick either 1st segment or MSC, AFAIK all carriers go by marketing carrier of 1st segment, not marketing carrier of MSC. (Most cases these are the same marketing carrier anyway)