Literature can be divided into first-class literature, second-class literature and third-class literature according to its content, nature and processing mode. A kind of literature refers to the original literature created on the basis of the author's own research results, such as periodical papers, research reports, patent specifications, conference papers, etc. Secondary literature is a retrieval tool produced by processing and sorting the primary literature, such as bibliography, title record, introduction, abstract and so on. The third-level document is a document compiled by comprehensive analysis on the basis of the first-level and second-level documents. People usually refer to these documents as the results of "information research", such as reviews, thematic reviews, annual summaries of disciplines, progress reports, data manuals and so on.
For example,
For example, Quan belongs to the first-class literature, and the catalogue index compiled by Modern People belongs to the second-class literature. If I write a summary of the study of springs in past dynasties, it belongs to third-class literature.