A four-digit SIC, or Standard Industrial Classification, code is used by the government to identify the primary business of the company. It was developed so that statistical reports across different government agencies can be understood easily.
The digits of a SIC code describe how a business is categorized. The first two digits identify the major industry group, while the third digit defines the general industry group, and the fourth digit identifies the exact industry. SIC codes were replaced by NAICS in 1997, but the United States Department of Labor has a search engine that can help look up SIC codes.