All original documents based on the author's own experience, research or development achievements are called primary documents after publication or exchange. First-class literature is the main body of literature, the most basic information source and the object of literature retrieval. Its characteristics are concrete, detailed and systematic, with opinions, facts and conclusions. Generally, it includes periodical papers, monographs, research reports, conference documents, dissertations, patent specifications, technical standards, technical files, scientific and technological reports, etc.

The secondary literature is formed by processing, refining and concentrating the primary literature. It transforms a large number of scattered and disorderly information into an orderly and manageable system. A document is compressed into items, also called items,

Reference,

Citation,

Records), and organized in a certain order, become a tool that can be used to find documents. Secondary literature, also known as second-hand information, includes retrieval tools such as catalogue, title record, abstract and index, which can report and guide the primary literature.

The third document is formed by collecting, synthesizing, analyzing and describing the first and second documents in detail. It includes manuals, encyclopedias, yearbooks, catalogues, guides and other reference books. There are also secondary literature catalogues, guides, comprehensive and important documents belonging to the category of tertiary literature.

The processing from first-class documents, second-class documents to third-class documents embodies the process of centralization and ordering of documents, while the demand for original documents by users is often a reverse process.