Categories such as Formula Three and FIA Formula 2 Championship are described as feeder formulae, which refers to their position below Formula One on the career ladder of single-seater motor racing. There are two primary forms of racing formula: the open formula that allows a choice of chassis or engines and the control or "spec" formula that relies on a single supplier for chassis and engines. Formula Three is an example of an open formula, while Formula BMW is a control formula. There are also some exceptions on these two forms like Formula Ford where there is an open chassis formula but a restricted single brand engine formula.