From the California commercial DMV website...

A commercial vehicle is a motor vehicle or combination used for hire to transport passengers or property or which:

Has a gross vehicle weight rating (GVWR) of 26,001 pounds or more
Is designed, used, or maintained for carrying more than 10 passengers, including the driver
Tows a vehicle or trailer, which has a GVWR of 10,001 pounds or more
Transports hazardous materials, which requires placards
Tows any combination of two trailers or vehicle and trailer

Cali DMV