If you mess up your labels, Amazon will reject your shipment. It's that simple.
1. UPC (Universal Product Code)
This is the code you buy from GS1. You need this to create the listing in the catalog, but you should NOT put this on your product packaging if you are using FBA.
2. FNSKU (Fulfillment Network Stock Keeping Unit)
This is "Amazon's Barcode". It starts with "X00...". You MUST start using FNSKU labels.
Why? If you use the UPC (Commingled Inventory), your real legit product gets mixed in a bin with fake counterfeits from other shady sellers. When a customer buys from "You", Amazon might grab a fake unit from the bin. You get the bad review.
3. GTIN Exemption
If you manufacture your own private label goods, you can apply for a GTIN exemption to list products without buying a UPC for every variation.