For IATA carriers the "most significant carrier" rule applies for connections, in your example the MSC is AC on the TATL leg. MSC went into effect a few months ago (April 2011), so not many pax have experience with it, and it is quite complex, so those interested can google the full rules -- rarely is the first leg the MSC, though it can be. Also US DOT has separate (more favourable) rules, which only apply for flights on US flag carriers or if O/D is in the USA (neither of which is the case for the OP).