A person who operates (i.e., drives) a Commercial Motor Vehicle (CMV) with a gross vehicle weight rating (gvwr) of 26,001 or more pounds; or is designed to transport 16 or more occupants (to include the driver); or is of any size and is used in the transport of hazardous materials that require the vehicle to be placarded.
(Definition from Office of Drug and Alcohol Policy Compliance)
FMCSA DOT Post-Accident Drug Testing Requirements