CVC Section 2813 outlines who must stop at weigh stations and inspection stations:
2813. Every driver of a commercial vehicle shall stop and submit the vehicle to an inspection of the size, weight, equipment, and smoke emissions of the vehicle at any location where members of the California Highway Patrol are conducting tests and inspections of commercial vehicles and when signs are displayed requiring the stop. Every driver who fails or refuses to stop and submit the vehicle to an inspection when signs are displayed requiring that stop is guilty of a misdemeanor.
CVC Section 260 defines "commercial vehicle" (bolding added):
260. (a) A "commercial vehicle" is a motor vehicle of a type required to be registered under this code used or maintained for the transportation of persons for hire, compensation, or profit or designed, used, or maintained primarily for the transportation of property.
(b) Passenger vehicles which are not used for the transportation of persons for hire, compensation, or profit and housecars are not commercial vehicles. ...
(c) Any vanpool vehicle is not a commercial vehicle.
CVC Section 410 defines a "motor truck" (bolding added):
410. A "motor truck" or "motortruck" is a motor vehicle designed, used, or maintained primarily for the transportation of property.
CVC Section 471 defines "pickup truck" (bolding added):
471. A "pickup truck" is a motor truck with a manufacturer's gross vehicle weight rating of less than 11,500 pounds, an unladen weight of less than 8,001 pounds, and which is equipped with an open box-type bed not exceeding 9 feet in length. "Pickup truck" does not include a motor vehicle otherwise meeting the above definition, that is equipped with a bed-mounted storage compartment unit commonly called a "utility body."