STANDARD INDUSTRIAL CLASSIFICATION CODE (SIC CODE NUMBER):

A four digit number assigned to identify a business based on the type of business or trade and manufacturing, while the last two digits correspond to subgroups such as constructing homes versus constructing highways. A business can determine its SIC number by looking it up in a directory published by the Department of Commerce, or by checking in the SIC book in the reference section of a local library. SBA size standards are based on SIC codes. 

Classification of businesses as defined by the U.S. Department of Commerce 

A four digit code that indicates a company's main line of business. Some companies operating in numerous lines of business will have secondary SIC codes. 

Each employer, upon application for a federal employer identification number (FEIN) is assigned a SIC number. The Standard Industrial Classification was developed for use in the classification of businesses by type of activity in which they are engaged as a way to promote uniform collection of data at the federal level. SIC codes are based upon the primary activity or output that a company is engaged in.