Why are Some SIC Code lists 8-Digit and Others 4 Digits?

U.S. SIC Codes are defined at the four digit level. Eight-digit SIC Codes are not official U.S. government SIC codes, therefore, historical statistical data cannot be found for any SIC Code beyond the four-digit level via government sources.  Six, seven and eight-digit codes have been created by private data companies to help further classify industries for marketing and identification purposes.