Here is the FMCSA definition:
Commercial motor vehicle means any self-propelled or towed motor vehicle used on a highway in interstate commerce to transport passengers or property when the vehicle—
(1) Has a gross vehicle weight rating or gross combination weight rating, or gross vehicle weight or gross combination weight, of 4,536 kg (10,001 pounds) or more, whichever is greater; or
(2) Is designed or used to transport more than 8 passengers (including the driver) for compensation; or
(3) Is designed or used to transport more than 15 passengers, including the driver, and is not used to transport passengers for compensation; or
(4) Is used in transporting material found by the Secretary of Transportation to be hazardous under 49 U.S.C. 5103 and transported in a quantity requiring placarding under regulations prescribed by the Secretary under 49 CFR, subtitle B, chapter I, subchapter C.
As Jeff stated not all states are in agreement with what constitutes a commercial vehicle. Signage prior to a weigh station may say:
ALL TRUCKS OVER (weight) MUST WEIGH. Key word here is TRUCKS. Is a van a truck?
ALL COMMERCIAL VEHICLES OVER (weight) MUST WEIGH. Key words here are COMMERCIAL VEHICLES. Note the lack of the word MOTOR between the words COMMERCIAL and VEHICLES.
States have different laws, wording, definitions and enforcement policies. I don't stop at scales unless I have HAZMAT on board, which I don't haul any more, so I don't stop at scales. If I should ever get pulled over I will go into my "ignorant" routine. It comes naturally!