California Code, Public Utilities Code - PUC § 5385

A charter-party carrier of passengers, except those engaged in the provision of a hired driver service when a rented motor vehicle is being operated by the hired driver, shall not operate any motor vehicle on any public highway unless there is displayed on the vehicle a distinctive identifying symbol in the form prescribed by the commission, showing the classification to which the carrier belongs.Such an identifying symbol shall not be displayed on any vehicle until a permit or certificate of public convenience and necessity under this chapter has been issued to the carrier. The identifying symbol displayed by charter-party carriers subject to the Interstate Commerce Commission shall serve in lieu of the display requirements of this section.