Literature can be divided into first-class literature, second-class literature and third-class literature according to its content, nature and processing mode. Primary 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.

First-class literature refers to the literature created or written by the author based on his own research results, regardless of whether he refers to or quotes other people's works when creating, and regardless of the material form in which the literature appears. Most articles published in journals and papers published in scientific and technological conferences belong to one document.

Secondary literature refers to the products obtained by the literature workers after processing, refining and compressing the primary literature, and is a tool literature edited, published and accumulated for the convenience of the management and utilization of the primary literature. Retrieval reference books and online retrieval engines are typical secondary documents.

The third document refers to the product of extensive and in-depth analysis and research on related first-class documents and second-class documents. Such as encyclopedias and dictionaries.