Category is the main body and basic component of enumeration taxonomy, and its function is to reveal a group of documents with the same attributes, that is, similar documents. It consists of three parts: class name, class mark symbol (see classification mark) and comment. There are many types of categories: ① According to the document attributes reflected by categories, they can be divided into content categories and form categories. Content category reflects the essential attributes of literature, that is, the content of literature, such as the subject, position, viewpoint, method, region, time, people and so on; The category of form reflects the non-essential attributes of literature, that is, the form of literature, such as form genre, language used, time and place of publication, version and binding, etc. ② According to the number of categories, it can be divided into single category and combined category. A single category only reflects a certain subject or a certain problem; Combinatorial category, also known as category group, reflects several disciplines or problems, such as "mathematical chemistry", "culture, science, education, physical education" and so on. ③ According to the relationship between categories, it can be divided into subordinate categories, parallel categories, alternating categories and related categories. Subordinate category refers to the category with subordinate relationship, such as "inorganic chemistry" and "chemistry". The category of juxtaposition refers to the category with juxtaposition and some categories with opposition and contradiction. Alternating categories are categories that reflect the same relationship, that is, some categories can belong to two or more superior categories and be listed in the classification table at the same time, but only one of them is selected in actual classification. Those that are selected are called use categories, and those that are not used are called alternate categories. Related categories are categories that reflect cross-relationships. ④ According to categories, it can be divided into basic categories, basic categories (first-class categories), second-class categories and third-class categories. The basic category, also known as the most part, is the most general and essential distinction first determined in the document classification table. Usually, it is not listed separately in the document classification table, and it is not used for actual classification. It is generally put forward in the preparation instructions. The basic category is the first category in the document classification table. For example, Chinese Library Classification has 22 basic categories, and Dewey Decimal Classification has 10 basic categories. The basic category is followed by the secondary category and the tertiary category.