It would apear to be "Sport", which is a bit overkill since sport is allready in the name Sport Wagon.
The new Kia Optima Sport Wagon Grand Touring Line Sport; sporty, spacious, smart, smooth and switched-on, according to the brochure at least
Perhaps GTS was just a bit too much for a family car so GT line S was chosen, the advertising blurb does say that the GT line S is a GT line with sportier trim.
They may have decided to just copy Audi with the S line sufix.