What do the 12 digits mean?
The first digits identify the manufacturer, the middle digits identify the specific product, and the last digit is a check digit. The check digit is calculated from the other 11 using a fixed formula, and it’s what lets a scanner catch a misread instead of ringing up the wrong item.
Can you just make up a UPC number?
You can generate a symbol from any 12 digits, but that doesn’t make the number valid for retail. A real UPC has to be issued to your business by GS1, which keeps manufacturer prefixes unique so two companies never end up with the same code on different products. A barcode generated from a made-up number will scan perfectly and still get a listing rejected on Amazon, eBay, or by a retailer, because the problem is the number’s registration status, not whether the symbol reads correctly.
When don’t you need a registered UPC?
For anything that isn’t a retail product listing. Internal stock tracking, warehouse locations, asset tags and your own inventory systems don’t require a GS1-registered number. Code 128 is usually the better choice for those anyway, since it isn’t tied to retail registration at all.