Originally Posted by
jimrpa
Many years ago, Delta was experimenting with a system whereby the lowest COACH fares (L, U, and T) would only qualify for 1/2 the MQMs (and mileage) of a flight. Hence, the mnemonic ".5LUT" fares came into being.

Ah! So it isn't an S, it's a 5... Now I get it.

Thanks for the history lesson.