California Code, Business and Professions Code - BPC § 104

All boards or other regulatory entities within the department's jurisdiction that the department determines to be health-related may adopt regulations to require licensees to display their licenses or registrations in the locality in which they are treating patients, and to inform patients as to the identity of the regulatory agency they may contact if they have any questions or complaints regarding the licensee. In complying with this requirement, those boards may take into consideration the particular settings in which licensees practice, or other circumstances which may make the displaying or providing of information to the consumer extremely difficult for the licensee in their particular type of practice.