There are two types of CDL licenses that are important to understand when renting a vehicle — Class A and Class B. (To learn more about the classes of commercial driver's licenses, visit the Federal Motor Carrier Safety Administration's [FMCSA] website. The requirement for a CDL comes down to these four letters: GVWR, which stands for Gross Vehicle Weight Rating. It's the maximum operating weight for a vehicle. A manufacturer determines its trucks' GVWR based on various specifications, including the type of engine, fuel, passengers, and its cargo-hauling capabilities. Most often, a vehicle lists a weight rating for its front and rear axles on the inside doorjamb. Add those two numbers together, and you get your vehicle's GVWR. Put simply, GVWR is the maximum loaded weight allowed, and the FMCSA has determined that any truck used to transport passengers or property with a GVWR of 26,001 lb. or more is a commercial motor vehicle.