Original documents
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.
The second article
Secondary literature is one of the retrieval tools such as bibliography, title record, introduction and abstract.
Three documents
The three documents are based on the first-level documents and the second-level documents. People usually refer to these documents as the results of "information research", such as reviews, thematic reviews, annual summaries of disciplines, progress reports, data manuals and so on.
Zero-order literature
Zero-level documents refer to original documents without any processing, such as experimental records, manuscripts, original audio recordings, original video recordings, conversation record, etc. Zero-level documents play an important role in the preservation of original documents, the verification of original data and the verification of original ideas (owners).