I've got AMS-LHR-PDX-SEA booked in U, and with PDX-SEA in S (no E availability)

Just checked and it's in S even while there is T availability (which is what I would expect it to book into by default). Though there was a schedule + flight change after the original booking, so possibly there wasn't T at the time I booked. At least S gets me premium class seats for free.