14 CFR § 65.103

A certificated repairman may perform or supervise the maintenance, preventive maintenance, or alteration of aircraft or aircraft components appropriate to the job for which the repairman was employed and certificated, but only in connection with duties for the certificate holder by whom the repairman was employed and recommended.

A certificated repairman may not perform or supervise duties under the repairman certificate unless the repairman understands the current instructions of the certificate holder by whom the repairman is employed and the manufacturer's instructions for continued airworthiness relating to the specific operations concerned.

This section does not apply to the holder of a repairman certificate (experimental aircraft builder) issued in accordance with § 65.104 or to the holder of a repairman certificate (light-sport) issued in accordance with § 65.107, while that repairman is performing work under that certificate.