As it is known in the USA, National Stock Number is a 13 digit number assigned to items supplied under the Federal Catalog Sytem. 

The typical sequence is XXXX-XX-XXX-XXXX

The NSN consists of the 4 digit Federal Supply Class (FSC) at the beginning and 9 digit National Item Identification Number or NIIN following the FSC.  The first 2 digits of the FSC are a broader Federal Supply Classification Group (FSG).  The first 2 digits of the NIIN are used to indicate the country that first codified the item.  For example, 00 and 01 are USA.  02-10 are unassigned. Many other countries including namely NATO countries recognize and utilize the NSN system as well.  The last 7 digits of the NSN is the unique number assigned to the item.