The National Stock Number (NSN) as it is known in the US, is a 13-digit numeric code, identifying all the ‘standardized material items of supply’ as they have been recognized by all NATO countries including United States Department of Defense. Pursuant to the NATO Standardization Agreements, the NSN has come to be used in all treaty countries.
On the Online NSN Catalog users ca narrow down the NSN Catalog into the different Federal Supply Classes (FSC).
The NATO Stock Number consists of the NATO Supply Class (NSC or FSC) and the National Item Identification Number (NIIN). However the NIIN alone uniquely identifies the item, the FSC merely adds context by indicating the general classification of the item.
On the above mentioned resource, http://www.nccparts.com/nsn-catalog.php, the first column is the FSC Base number of an NSN. The second column is the number of items that are categorized under the FSC Base. And the last column is the title of the category for each of the FSC’s.