Class is the highest level, and indicates the overall type of tool or machine. Category represents various sub-types of the class, Type represents various sub-types of the category while Subtype represents the lowest level of classification.

Note that the contents of these lists are context-sensitive, i.e. the items in the Category list depend on what is selected in the Class list.

The classifications can be changed over time. The current hierarchy and patent counts can be seen here.