Can’t help you on the exact language of service agreements on US / Canada air services, but the US is (as mentioned earlier on this thread) really down on the idea on Canadian carriers ‘stealing’ domestic US traffic. So there are no sixth freedom thru routings at all, with a Canadian carrier starting in the US, stopping in Canada, then returning to the US. Even more, it is extremely difficult to book a journey from one US point to another US point via Canada. Try booking Los Angeles to Boston via Toronto with Air Canada and you’ll see that the booking engine simply won’t accept it. Even intermediaries (such as Expedia) won’t allow such bookings. A wonderful illustration of how the US, great chamption of free trade that it pretends to be, erects non-tariff barriers to free competition. Hope this helps you.