North American Industry Classification System. A code number system used to identify various types of industries. This code system replaces the SIC (Standard Industrial Classification) code system used prior to 1997. Use of these code numbers is often mandatory. Some companies have several processes, which will cause them to fit into two or more classifications. The code numbers are available from NTIS, 5285 Port Royal Road, Springfield, VA 22161. NTIS Stock No. PB2007-100002.