The California Air Resources Board, also known as CARB or ARB, is the Clean Air Agency in the government of California. Exempted parts are add-on or modified parts that have undergone an ARB engineering evaluation. If the part or modification is shown not to increase vehicle emissions is granted an exemption to emission control system anti-tampering laws. This exemption is called an Executive Order (EO) and allows the modification to be installed on specific emission-controlled vehicles. Every Executive Order part or modification has an assigned number that Smog Check stations, BAR Referee stations, or the ARB can verify.