California Code, Public Utilities Code - PUC § 5384

(a) Specialized carriers, who do not hold themselves out to serve the general public, but only provide service under contract with industrial and business firms, governmental agencies, and private schools or who only transport agricultural workers to and from farms for compensation or who only conduct transportation services, which are incidental to another business.This permit shall be designated as a “Z” permit.

(b) Carriers using only vehicles under 15-passenger seating capacity. This permit shall be designated as a “P” permit.

(c) Carriers conducting round-trip sightseeing tour service.This permit shall be designated as an “S” permit.