From my experience, S is a code for staff tickets. Last year I was lucky enough to win an EY ticket and it was booked as S. Didn't know this until I've checked in at biz class counter as an EY Gold. Was told by the agent that I shouldn't be checking in there. Also on my way back, was refused entry to my flight to LHR which I was told full. Anyway at the end of the day it was a free ticket and I've had a great trip!