Originally Posted by
Tomasz Pa
so it should be X not Y.
Under the old program, yes.
In the new program, some AC award space is available to partner programs at a cost fixed by those programs' award charts. The economy seats made available in this way are "X space", because that's the booking code for "seats AC allows to be booked with partner miles".
...but above and beyond that inventory, (theoretically) every AC flight is now available to book with a dynamically-priced amount of Aeroplan miles, in various fare types (Economy Standard, Flex, Latitude, etc) and these won't generally be coded as X. For example, my last domestic short-haul booked on points in Economy Standard was coded as K on the outbound and S on the return, very similar to a revenue ticket.
It's a moot point in this case, though, since the Q400 that'll be flying you to Seattle doesn't have a business cabin. There's nothing to upgrade to.