Core documents usually refer to some documents closely related to the development level and trend of this discipline. Related literature and marginal literature refer to the literature whose content is relatively far from the theme. From the background of discipline development, related documents and marginal documents are the result of mutual infiltration and combination of technology between this discipline and other disciplines. 1934, British chemist and linguist S.C. Brandford made an experiment. According to the number of papers, it is found that 1/3 of all papers on electrical technology are only published in professional journals of this discipline. Other 1/3 papers appeared in about 50 "related" journals that have no direct relationship with electrical technology, such as Energy Science and Transportation. Finally, 1/3 papers were published in more than 200 journals unrelated to electrical technology. On the basis of statistics, he summed up the following rules:

Among all the journals that publish papers on a certain subject, there is the first area, that is, the core area, which consists of several journals. There are two other areas around it, and the number of papers in each area is the same as that in the core area. Then the number of periodicals in the second region is n times that in the core region, while the number of periodicals in the third region is n2 times that in the core region. In this way, the number of periodicals in the core area and the second and third areas will be 1:n:n2, where N≈5.

Core documents usually refer to some documents closely related to the development level and trend of this discipline.