Originally Posted by
global happy traveller
It depends on how the two airlines map their fares (which is not very well publicized). See the AC forums and there are many instances where AC J and PY TPAC codeshares are mapped to Y class or the fares are mapped to another airline's lower earning mileage etc.
It's absolutely possible for UA to map its W fare class to AC's K fare class behind the scenes, which is why I asked if these were UA codes or AC codes.
However, that has nothing to do with Tango. AC no longer determines Tango based on fare class. All fare classes can have Tango fares; Tango or Flex is coded into the fare, not the booking class. Looking at ORD-YUL, for example, I see that AC has filed Tango fare L21S1TG, Flex fare L21S1FL, and Comfort fare L21S1CO; all of these book into L class.