A category is the aggregate of classes formed by differentiation within a population on a single basis. A category includes the entire population. It includes all classes differentiated on one basis. It is distinguished from a class, which is only one part of a category and is definable only in terms of the basis of differentiation. The higher categories of a hierarchy have fewer classes and they are more inclusive than the classes of the lower categories that include attributes from all the higher categories. Thus, high categorical levels are associated with high-level generalizations or abstractions. These abstractions are used as the bases of differentiation, but these ideas are expressed in terms of attributes assumed to be their consequences. Thus, soil attributes thought to be the results of such processes are the criteria used to segregate soils.