From the State Bar website:
What is a California certified legal specialist?
California attorneys who are certified as specialists have taken and passed a written examination in their specialty field, demonstrated a high level of experience in the specialty field, fulfilled ongoing education requirements and been favorably evaluated by other attorneys and judges familiar with their work.
Only these attorneys can identify themselves as certified specialists in California because they are the only attorneys who are certified either by the California Board of Legal Specialization or by an organization whose certification program has been accredited by the State Bar. Such an organization must have requirements for certification that are at least equal to those of the State Bar's program.