It is a retrieval language that classifies and arranges a large number of concepts that express the contents of literature information and retrieval topics according to their disciplinary nature, and becomes a logical system that basically reflects the usual classification system of scientific knowledge, and uses numbers (classification numbers) to represent concepts and their positions in the system, and even to express the relationship between concepts. China Library Classification is the basis of book classification in China. China Library Classification divides all knowledge categories into Marxism-Leninism and Mao Zedong Thought according to the five-point method. Philosophy; Social science; Natural science; Five categories of comprehensive books. On this basis, a system series consisting of 22 categories is constructed.

(2) Theme language

Refers to the controlled words that express the information content of documents. Subject words need to be standardized, and thesaurus is the embodiment of the language of subject words. The words in thesaurus are used as the basis for identifying the content of documents and finding documents.

(3), keyword language

Refers to the keywords extracted from the document content as the identification of the document content and the basis for searching the directory index. There is no need to standardize keywords, and there is no need to use keyword list as a tool for indexing and searching books and materials.

(4) Natural language