There are various systems used to classify business entities. They are used for statistical purposes and in databases as part of the indexing. Knowing these schemes can make industry research a bit easier because most databases, many of the print sources, and the U.S. government (most notably the Census) use them to create charts, graphs, lists, and indexes.
This site is maintained by Raymond Robertson, Associate Professor of Economics at Macalester College, and it provides links to a number of concordances between various systems of industry classification as well as brief descriptions of these systems.
"S.I.C. Pursuits: The Consequences and Problems of Classifying Establishments for Government Statistic," by Rolf R. Schmitt and Michael Rossetti. In Proceedings of the Urban and Regional Information Systems Association: 1987, Volume IV, pp. 15-24.
Online edition at Bureau of Transportation Statistics
This review, prepared by the representatives of the U.S. Department of Transportation to the Technical Committee on Industrial Classification in connection with the Standard Industrial Classification System (SIC) code revision, provides a brief overview of the SIC System and discusses issued raised in relation to its revision.
Ryan L. Phillips & Rita Ormsby (2016) "Industry classification schemes: An analysis and review," Journal of Business & Finance Librarianship, 21:1, 1-25
This article examines the industry classification schemes used in business research including the Standard Industrial Classification (SIC) the North American Industry Classification System (NAICS) to market-based industry classification schemes like the Global Industry Classification and the Industry Classification Benchmark. The examination answers why there are so many, who developed them, and for what purpose. The article also introduces the new Sustainability Industry Classification System External from the Sustainable Accounting Standards Board.