California regulates aftermarket motorcycle parts that have the potential to impact emissions. In most cases, the sale and use of emissions related aftermarket parts on motorcycles is prohibited unless it is either a "replacement part" as defined by California, or is a specifically authorized use of that part as reflected in an Executive Order ("EO Part.") Competition Use Only Parts may not be sold or used on a motorcycle that is used on or off the public highways other than those motorcycles used exclusively for competition. Competition Use Only Parts are aftermarket parts that replace or otherwise interfere with the operation of an emission control device, such as a catalytic converter or oxygen sensor, and may be sold and used on a motorcycle that is used only for closed course competition.