generaly there are three classes of trucks,light,medium and heavy.light trucks are 1 ton max accept for some military government spec trucks.anyway medium duty starts at 2 tons or class 4 and ranges to class 8 from there are medium-severe duty and heavy duty wich are usually semi tractors or cement mixers etc.each class is designated and specified by weight range wich factors in axle,suspension and tire load range.other components are specific to power and torque such as drive lines, input, and output shafts in the tranny etc.