The primary literature is the original literature, the secondary literature is the product of processing and sorting the primary literature, and the tertiary literature is the product of comprehensive summary after extensive and in-depth analysis and research on the primary literature and the secondary literature.

In information science, different terms are often used to describe different levels and attributes of documents. A document is an original document containing inventions and new ideas. This includes periodical papers, research reports, patent specifications, conference papers, dissertations and technical standards. The secondary literature records the external characteristics of the primary literature, such as the title, author and source, and compresses the content of the literature into the form of introduction, abstract or abstract. The purpose of secondary literature is to sort out and classify the primary literature in order to find and use it easily. The third document is the product of comprehensive generalization after extensive and in-depth analysis and research on the first document and the second document. It is the result of comprehensive research and summary of relevant literature, including review, thematic review, annual summary of disciplines, progress report, data manual, progress publications and literature guide.