The National Stock Number consists of a four-digit Federal Supply Classification (FSC) code number plus a nine-digit National Item Identification Number (NIIN).

The FSC class code number indicates the commodity group and subgroup to which the item belongs.

The NIIN consits of a two-digit National Codification Bureau (NCB) code ("00" or "01" for US), and a seven-digit nonsignificant number.
The NIIN provides positive identification of the item (a given NIIN is assigned to one NSN only, and is never duplicated or reused).

Reference

• "The National Stock Number: the Gear That Keeps the Supply Chain Running" [pdf]