49 CFR §390.33

Whenever a commercial motor vehicle of one type is used to perform the functions normally performed by a commercial motor vehicle of another type, the requirements of this subchapter and part 325 of subchapter A shall apply to the commercial motor vehicle and to its operation in the same manner as though the commercial motor vehicle were actually a commercial motor vehicle of the latter type. Example: If a commercial motor vehicle other than a bus is used to perform the functions normally performed by a bus, the regulations pertaining to buses and to the transportation of passengers shall apply to that commercial motor vehicle.