I don't know if it stands for anything other than Special .. but what it gets you is the burger with the lot from the TSA. They check everything short of a strip search. SEA TAC will give you a pat down but SFO has some sort of machine ( the air puffer ). They then go through all your hand luggage with explosive detecting wipes .. even to the point of opening our Senheiser headphone bags. It's a little annoying but it's all part of modern travel.
When we got our first one I thought someone had done us a favour to get through the queue faster ... turns out I was wrong