49 CFR § 523.10 - Heavy-duty trailers.
(a) A trailer means a motor vehicle with or without motive power, designed for carrying cargo and for being drawn by another motor vehicle as defined in 49 CFR 571.3. For the purpose of this part, heavy-duty trailers include only those trailers designed to be drawn by a truck tractor excluding non-box trailers other than flatbed trailer, tanker trailers and container chassis and those that are coupled to vehicles exclusively by pintle hooks or hitches instead of a fifth wheel. Heavy-duty trailers may be divided into different types and categories as follows:
(5) The following types of equipment are not trailers:
(i) Containers that are not permanently mounted on chassis.
(ii) Dollies used to connect tandem trailers.
(iii) Equipment that serves similar purposes but are not intended to be pulled by a tractor.
(b) Heavy-duty trailers do not include trailers excluded in 49 CFR 535.3.