File classification:

1. Original literature: original literature, usually referring to the author's originality. This kind of literature is specific, systematic, detailed, novel and creative, and it is the main object of literature retrieval. Monographs, periodical papers, dissertations, research reports, conference papers, patent specifications, etc. All belong to the same literature.

2. Secondary literature: commonly known as retrieval tools. It is formed by compressing a document and collecting, sorting and processing a scattered document. Because it provides clues and convenience for searching and using first-class literature, it is regarded as the main tool and means of literature retrieval. Title, table of contents, index, abstract, etc. They are all secondary documents.

3. Third-level literature: Generally, on the basis of using a large number of second-level literature, a large number of first-level literature are selected, read and studied, and concentrated and refined. Three documents or comments on achievements, progress or predicted development trends, so that readers can understand the current research level and trends. Therefore, it is considered as an advanced intelligence. Yearbook, progress, review and summary all belong to three documents.