That have me the idea to look up where they get that information and I found it.
46 CFR 136.105 Applicability.
(a) This subchapter is applicable to all U.S.-flag towing vessels as defined in §136.110 engaged in pushing, pulling, or hauling alongside, except —
(4) A seagoing towing vessel of 300 gross tons or more subject to the provisions of subchapter I of this chapter;
(5) A vessel inspected under other subchapters of this chapter that may perform occasional towing;