Contents, indexes and abstracts belong to secondary literature. Summary, comments and comments belong to three documents.

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 is a retrieval tool, such as bibliography, title, introduction, abstract and so on. The third generation literature comes first. Documents compiled after comprehensive analysis are usually called the results of "information research", such as reviews, thematic reviews, annual summaries of disciplines, progress reports, data manuals, etc. Similarly, some people divide information into primary information, secondary information and tertiary information.