When I was pricing out SEA-SIN for Sept and got a low priced Q, it was a "QLXSCDCM", and the 1K agent said that the "CM" at the end (or maybe it was the SCDCM?) said it was an internet-only fare that was not eligible for upgrade with SWU. Incidentally, this is a fare that shows up with "Everyday Leisure Fares",