The UPC symbol has two parts:
The machine-readable bar code and The human-readable 12-digit UPC number
The manufacturer identification number is the first six digits of the UPC number — 753182 in the image above. The next five digits — 95342 — are the item number. The GS1 also supplies 7, 8 and 9 digit manufacturer numbers as well.
A person employed by the manufacturer, called the UPC coordinator, is responsible for assigning item numbers to products, making sure the same code is not used on more than one product, retiring codes as products are removed from the product line, etc.
Typically, every item that a manufacturer sells, in addition to every variation of the item requires a different item code.
Since the barcode is also used to track inventory, it is important to have a different barcode for each of these variations. Using shoes as an example, a man’s oxford shoe may come in Black, Brown, Cordovan, each in sizes, 9, 10, 11, 12, 13 and 14. Each of these variations (3 colors x 6 sizes = 18 different products)