Group CN is a category of motorsport, introduced by the FIA in the early nineties for sports car racing. Group CN cars are mainly seen in hillclimbing championships or sports racing series. Group CM is a non-FIA class which is closely related to Group CN.
Group CN Wikipedia
Group CN rules are applied to a variety of championships. Many championships alter the rules in their own way like the Radical European Masters and V de V championships.
The Radical European Masters is a spec racing series in Europe. It features the Radical SR8, Radical RXC Spyder and Radical SR3, each in their own class.
The Speed Euroseries is an open sports racing class based in Great-Britain. There is a free choice of chassis, like Norma and Ligier. All cars are powered by the Honda K20A engine.
The French-based V de V organisation features two FFSA Group CN based classes. The endurance championship is open for any chassis and engine within the regulations. The V de V organisation also features the Funyo Challenge open to Peugeot powered Funyo 4 or Funyo 5.
A Group CN racing class exists in many countries. Examples are the Radical UK Cup which features the Radical SR3 and the Open España Prototipos.