The materials that can be used for reference are always called documents, such as real historical records and military records. Secondary documents are search reference books, such as various types of dictionaries and reference books. Online search engines are still called secondary documents, such as things found by 360 browsers and things found by Baidu (these things are also called secondary documents because they have original and true records, and some people publish them to these search engines on network terminals or websites, so that we can get the results we need. On the basis of primary literature, this kind of search is called secondary literature). The third document is the result of synthesizing the first document and the second document, and then carrying out a lot of analysis and research. I made myself clear, huh? ! Hey hey. . .