Originally Posted by D582

There is no need to panic, this is not a Tango TATL fare. The booking class letter is what matters, and in this case for the YYZ-ZRH flight it is T (which is Tango Plus). I have determined what you are seeing:

AC's website is breaking the fare in YYZ for that particular departure time, that is why Tango is an option. You will note in the fare rules 3 separate fare basis codes:

From: Montreal, QC (YUL - Trudeau)
To: Toronto, ON (YYZ - Pearson Int'l)
Fare Basis Code: P5TGB

From: Toronto, ON (YYZ - Pearson Int'l)
To: Zurich, (ZRH - Zurich)
Fare Basis Code: THRCT47

From: Zurich, (ZRH - Zurich)
To: Montreal, QC (YUL - Trudeau)
Fare Basis Code: THRCT47

It's doing this because there is no availability in T class on AC421, and likely the T class fare YUL-ZRH doesn't allow the domestic flight to be booked in a class higher than T. The next valid Tango Plus fare is in W class and is about $900++ R/T, so the website figured out it was cheaper to book a YUL-YYZ fare, then a YYZ-ZRH fare, instead of a through fare. If you select the 'Tango Plus' option for the 17:00 departure from YUL, it does the same thing, except books a Tango Plus fare (S5TPB) for the YUL-YYZ segment.