Maybe this will help:
Or this;
Subpart F - Vehicle groups and endorsements
§ 383.91Commercial motor vehicle groups:
(1) Combination vehicle (Group A)—Any combination of vehicles with a gross combination weight rating (GCWR) of 26,001 pounds or more provided the GVWR of the vehicle(s) being towed is in excess of 10,000 pounds.
(2) Heavy Straight Vehicle (Group B)—Any single vehicle with a GVWR of 26,001 pounds or more, or any such vehicle towing a vehicle not in excess of 10,000 pounds GVWR.
(3) Small Vehicle (Group C)—Any single vehicle, or combination of vehicles, that meets neither the definition of Group A nor that of Group B as contained in this section, but that either is designed to transport 16 or more passengers including the driver,
or is used in the transportation of materials found to be hazardous for the purposes of the Hazardous Materials Transportation Act and which require the motor vehicle to be placarded under the Hazardous Materials Regulations (49 CFR part 172, subpart F).
These are the Federal reg's and the above is a generalization. Not included are the regulations concerning recreational vehicles and such. MOST of the time, they are exempt. States can make the standards different from what the Fed's require but they cannot omit anything in the standards.