There are many classification codes, and sadly, they overlap in part only, never completely. They also have different principles, and different purposes! Here are some of them:
Some are national or regional, others are more global.
For more detail than is on this page, see these webpages:
– For quick look-ups: use Business Insights Essentials (BIE) or Hoover's Company Records
– For international scope: use Business Insights Essentials, Hoover's or Nexis Uni
– For comprehensive US coverage of both public and private companies: Mergent Intellect (also covers Canadian companies)
– For creating downloadable lists of companies: Mergent Intellect or Nexis Uni.