CDI (Capacitive Discharge Ignition) for outboard marine motors is basically a refined version of the old magneto ignition system, except the points have been replaced with electronic circuitry. CDI is used on some late model marine engines, and some late model motorcycle engines.
Do not use a standard resistor type plug on a motor with this type of Capacitive Discharge Ignition as it will cause misfiring and poor performance. Rather, be sure to use a plug with an inductive type resistor (such as the NGK Z-type). Use of non-inductive resistor type plugs on these motors can create an open circuit within the spark plug (killing the plug).
Modified automotive racing motors also have a form of CDI, (such as Mallory, MSD, Crane and Accel), these should not be confused with the marine or power sport systems.