NAICS uses a six-digit coding system to identify particular industries and their placement in this hierarchical structure of the classification system. The first two digits of the code designate the sector, the third designates the subsector, the fourth digit designates the industry group, the fifth digit designates the NAICS industry, and the sixth digit designates the national industry. The first revision of NAICS was for 1997, and this is the classification used for the 1997 benchmark I-O table. The upcoming 2002 benchmark table will be classified according to the 2002 NAICS. Please refer to http://www.census.gov/epcd/www/naics.html for additional information on the 1997 and 2002 NAICS.