The NSN (National Stock Number) is the 13-digit number that the federal government assigns, for purposes of identification and inventory control, to every piece of supply, equipment and material that it uses and buys. You can think of the NSNs as a federal cataloging system based on the concept of one NSN for any one item and one single item manager for each particular class of product. (Note that, because services are not inventoried, services don't fit this model. For services, only the first four digits are used. See the discussion of FSCs, above.)