Under U. S. Federal Motor Carrier Safety Regulations, a Commercial Drivers Licence (CDL) is required... .
Section 383. 3: (a) The rules in this part apply to every person who operates a commercial motor vehicle(CMV) in interstate, foreign, or Intrastate commerce, to all employers of such persons, and to all states.
Section 383. 5: Commercial Motor Vehicle (CMV) means a motor vehicle or combination of motor vehicles used in commerce to transport passengers or property if the motor vehicle-
(a) Has a gross combination weight rating of 11,794 kg or more (26,001 lbs or more) inclusive of a towed unit with a gross vehicle weight rating of more than 4,536 kg (10,000 lbs); or
(b) has a gross vehicle weight ration of 11,794 or more kg (26,001 lbs or more); or
(c) Is designed to transport 16 or more passengers, including the driver; or
(d) Is of any size and is used in the transportation of materials found to be hazardous for the purpose 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).
That is the fed rules and definition when a CDL is required. Michigan has a special license endorsement of (R) for Recreational Double. That is for the 5er guys pulling a second trailer behind the 5er.