California Code, Vehicle Code - VEH § 468

The department shall commence the “permanent trailer identification plate program,” on or after December 31, 2001, and may designate the method, consistent with this code, to be used by trailers, as defined in Section 5014.1, to receive an assigned permanent trailer identification plate for all trailers, except for trailer coaches and park trailers as described in subdivision (b) of Section 18010 of the Health and Safety Code, for identification purposes. An auxiliary dolly or tow dolly may be assigned a permanent trailer identification plate. The plate shall be in a size and design as determined by the department.