California Code, Vehicle Code - VEH § 4004

(a)(1) Commercial motor vehicles meeting the registration requirements of a foreign jurisdiction, and subject to registration but not entitled to exemption from registration or licensing under any of the provisions of this code or any agreements, arrangements, or declarations made under Article 3 (commencing with Section 8000) of Chapter 4, may, as an alternate to registration, secure a temporary registration to operate in this state for a period of not to exceed 90 days, or a trip permit to operate in this state for a period of four consecutive days.

(2) Each trip permit shall authorize the operation of a single commercial motor vehicle for a period of not more than four consecutive days, commencing with the day of first use and three consecutive days thereafter.  Every permit shall identify, as the department may require, the commercial motor vehicle for which it is issued.  Each trip permit shall be completed prior to operation of the commercial motor vehicle on any highway in this state and shall be carried in the commercial motor vehicle to which it applies and shall be readily available for inspection by a peace officer.  Each permit shall be valid at the time of inspection by a peace officer only if it has been completed as required by the department and has been placed in the appropriate receptacle as required by this section.  It is unlawful for any person to fail to comply with the provisions of this section.