I just flew an AC segment on a UA ticket this weekend. It was United fare class S, YYZ-SEA, which earned 100% status miles for all United segments. The AC segment between PDX and YYZ earned only 25% non-status miles ... despite no Tango fares being sold by Air Canada on that route. So, for this particular trip, United S must have mapped to a Tango fare. Lesson learned ... I should have booked AC directly into a Tango Plus ... or have flown entirely on United to avoid this disappointment.