Steve Bentley MD of Sofema Aviation Services www.sassofia.com – explains

A type certificate holder (TCH) can produce parts for its airplanes through the process of the manufacturers authorisation FAR Part 21 / EASA Part 21 Subpart G has the authorization to manufacture those parts on the basis of its type certificate, and production certificate.

However organisation who are authorised as PMA-holding manufacturers are permitted to make replacement parts for aircraft, even though they are not the original manufacturer of the aircraft. An applicant for a PMA applies for approval from the FAA.

An STC is a certificate. It defines the product design change, states how the modification affects the existing type design, and lists serial number effectivity. It also identifies the certification basis, listing specific regulatory compliance for the design change. Information contained in the certification basis is helpful for those applicants proposing subsequent product modifications and evaluating certification basis compatibility with other STC modifications.