FS - base model. Trigger action is DA/SA. That is, double action on first pull (when starting with hammer down) and single action thereafter. The decocker also acts as the safety. The "S" means that there is an added safety block that keeps the slide from flying off from recoil - a response to Army trials.
G - As above, but decocker does not act as safety. After decocking, the lever returns to position.
D - Double action only. No lever on slide.
