Actually, FAR Part 135:
Titled "Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons On Board Such Aircraft," and applies to turbojet engine powered aircraft with 1-30 seats, non-transport category turbo-propeller powered aircraft with 10-19 seats, and transport category turbo props with 20-30 seats.
Part 121 and Part 135 are Operating Rule parts ... it is Part 119 that governs the certification of these operations.