This post is useful: http://www.flyertalk.com/forum/briti...l#post17448077

And this even more so ...http://www.cxagentsusa.com/images/30...GE_RULES_1.pdf

Going by that .pdf I was incorrect.

With the USA DoT exception, CX are indicating the MSC is to be ascertained on the entire journey irrespective of Stopovers, so it's the marketing carrier of either first crossing between TC's, or if none then the first crossing between sub areas and if neither of these it's the first International segment.