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. Secondary literature refers to the processing of primary literature.
Dissertations, scientific reports and patent documents are first-class documents; Most periodical papers and conference papers belong to one document.
Contents, titles, abstracts and indexes belong to secondary literature.
Monographs, reviews, encyclopedias, standard documents and product samples belong to three kinds of documents. The comprehensive articles in periodical papers and conference papers belong to three times. A small part of the paper is a comprehensive paper, which also belongs to three documents.