My friend was flying in economy last week and had ST as well (waw-doh-bkk) on Qatar metal. I upgraded her on one of the return legs, tried to do the upgrade at the airport with qcredits for the outbound (waw-doh) but her class (W) didn’t allow that. Tried one more time at DOH first class lounge desk and they informed us she was already upgraded for free for doh-bkk. So my indication is that it might mean Single Traveller, and appears in case of possible overbooking/aircraft type change (waw-doh was rubbish to be B787, finally was A320) -and makes it easier for the airline to manage upgrades or denied boarding.