Depends on the breadth of the question and whether you truly mean a SAS that just addresses short-term rate damping? Probably the first point is that most aeroplanes without swept wings do not need a SAS as they rely on natural stability. Swept wing aircraft will typically have a yaw damper to counteract Dutch Roll, but nothing for pitch and roll axis, whereas a helicopter SAS will have stabilisation in pitch, roll and yaw as there is no natural dynamic stability. More advanced ones may also include the collective and have gust response modes to enhance hovering.