Why does the intermodal equipment provider (IEP) rule (73 FR 76794) refer to intermodal equipment and trailers, rather than by the common term "chassis?"

Most intermodal containers are moved by chassis, but some specialized types (such as tanks in frames) could be transported on drop-deck or other trailers. If such trailers are tendered to transport intermodal containers, they are subject to the provisions of the rule.