I''m pretty sure that AC flights cannot carry domestic U.S. traffic over a Canadian point on a routing such as BOS-YYZ-SEA. I would guess the same prohibition would apply to US or UA code-share traffic on AC metal, although I''m not sure what happens on US or UA metal. But regardless of the possible legality of any of the above scenarios, I don''t think too many passengers would use such a connecting service due to the need to go through both U.S. and Canadian Customs in each direction (assuming the passenger is making a round trip). At least, that''s how I understand that the system works, although I''ll defer to anyone with more expertise.