Although UA specifies that some AC discount fars do not accrue EQM's and accrue only 50% RDM's for flights within Canada, the truth is that this rule applies
for transborder as well as domestic flights.
The info below comes from the AC FT forum (
FAQ - booking classes) and applies to Aeroplan, but it appears that they apply the same rules when crediting UA miles.
TANGO
These are booked in R, N, G, P, E, T booking class.
They earn 50% Aeroplan miles but they do not count towards your status (re)qualification.
It would help to have official word from UA on what fares fall into which category, but is clear that many US-Canada fares do not qualify for EQM's and full RDM credit. See
United Reneges On Air Canada Mileage and
AC non-status miles for details.