California Code, Health and Safety Code - HSC § 43834

(a) The state board shall establish standards or criteria for the certification of auxiliary gasoline fuel tank evaporative loss control devices or systems on vehicles which are required, pursuant to this part or the National Emission Standards Act (42 U.S.C., Secs. 1857f-1 to 1857f-7, inclusive), to be equipped with a fuel system evaporative loss control device to prevent as much evaporation of gasoline into the air from auxiliary fuel tanks as is technologically feasible.

(b) For the purpose of this section, and Section 27156.1 of the Vehicle Code, an “auxiliary gasoline fuel tank” is a fuel tank which is designed and intended by its manufacturer for installation on, or which is installed on, a vehicle operating on gasoline and which is connected to the original fuel system, as defined in Section 39032 of this code, but is not a gasoline fuel tank which is added to a certified device on a used vehicle if such certification included the capability of handling evaporation from such a tank.