Catalogs, indexes, and abstracts belong to secondary literature. Reviews, commentaries, and commentaries belong to the tertiary literature.

Based on the content, nature and processing of the literature can be distinguished into: primary literature, secondary literature, tertiary literature. Primary literature refers to the author's own research results based on the creation of the original literature, such as journal articles, research reports, patent specifications, conference papers. Secondary literature is the primary literature after processing and organizing a class of aspects, such as bibliography, title, introduction, abstracts and other search tools. Three literature is in one, on the basis of the secondary literature, after a comprehensive analysis of the literature prepared, people often refer to this type of literature as "intelligence research" results, such as reviews, thematic reviews, annual summaries of disciplines, progress reports, data manuals. Similarly, there is also a distinction between intelligence into a primary intelligence, secondary intelligence, three intelligence.