Automatic Activation Device. Can be mechanical or digital. At a preset altitude if you are going fucking fast ie 70 mph it cuts the reserve closing loop. These are good if you are skydiving while inebriated or unconcious. Digital (the Cypres) are the best and rarely if ever fail when they are needed.

Analogue-Analogue-Digital. You might see this on the back of a CD which was originally mastered to analogue tape from an analogue source.