14 C.F.R. § 291.20
The rules in this subpart apply to cargo operations in interstate air transportation performed by air carriers certificated under sections 41102 or 41103 of 49 U.S.C. Subtitle VII. Section 41103 carriers that operate passenger-only or combination aircraft under section 41102, part 298 of this chapter, or other Department authority, must comply with the rules in this subpart in connection with cargo operations in interstate air transportation, whether provided on all-cargo or combination aircraft, operated pursuant to this authority or otherwise. In case a carrier may operate a particular flight under either a section 41102 certificate or a section 41103 certificate, the flight is presumed to be operated under the carrier's section 41103 authority.
14 C.F.R. §291.20