Class A CDL holder can drive a commercial motor vehicle with a gross vehicle weight rating (GVWR) of 26,001 or more that is towing a trailer that is 10,001 or more pounds. This is commonly called a combination vehicle. Class A CDL holders can drive all vehicles including Class B.
The difference between Class A and Class B licenses lies in the weight of the vehicles driven and/or towed. While a CDL is a license for vehicles that have a GVWR of over 26,001 or more, each class has a specific type of vehicles that a holder is legally allowed to drive.