The “gross combination weight rating” definition is being redefined by the FMCSA to help clarify when CDL and other safety regulations will apply to single-unit trucks that are towing trailers. The definition of GCWR needed to be updated to streamline the determination of whether or not a "combination driver" is prone to federal safety requirements. The GCWR definition is being revised because it has been too difficult for authorites to determine what is a commercial vehicle and when a CDL is required because the GCWR is not clearly labeled and/or easily found on the majority of trucks and trailers. "The new definition will classify GCWR as whichever of the following is greater:

A value specified by the manufacturer of the power unit if displayed on the Federal Motor Vehicle Safety Standard certification label required by the National Highway Traffic Safety Administration or
The sum of the GVWRs or the gross vehicle weights GVWs of the power unit and the towed unit(s), or any combination thereof, that produces the highest value."

You can read more by clicking here.