A magneto is device similar to an alternator for creating electrical energy. Many small gas engines, including several road worthy motorcycles, use them to generate electrical energy. They can be used for supplying spark or charging a battery.
I am guessing that efficiency (i.e. cost or power demand) plays the major role in deciding to use a magneto or an alternator. When is a magneto more efficient than an alternator? Is there some general rule of thumb used to decide which to use on an application?