Amazon's proprietary FNSKU barcode marks a unit as belonging to a specific seller's inventory pool, distinct from the manufacturer's UPC that ships on wholesale/arbitrage products. Only one barcode may be visible per unit, so sellers must cover or scratch out any existing UPC before applying an FNSKU label.
The mechanism matters beyond Amazon's stated preference: using FNSKU instead of UPC structurally prevents inventory co-mingling with other sellers listed against the same product — protecting a seller from receiving damaged, missing, or counterfeit units mixed in from someone else's stock in the same storage bin. Private-label sellers who need a UPC must buy one from GS1; Amazon can invalidate listings and pull ASIN-creation or selling privileges over third-party-purchased UPCs.
Applies only to FBA inventory physically stored in Amazon's warehouses.
The UPC side of this separation is sourced from GS1 and is what's used to actually create the Amazon listing in the first place. FNSKU generation happens later, after Brand Registry is approved: download the FNSKU barcode from Seller Central and send it to the supplier so they can label individual units before shipping to FBA.
Apply: sequence is GS1 UPC → create listing → get Brand Registry approved (see Trademark Registration Workflow (Pre-Sale)) → generate and send FNSKU to supplier for unit labeling — don't wait on Brand Registry to create the listing itself.
The UPC/GTIN side of this pair originates from a GS1 barcode, ordered via gs1.org before the Amazon listing is created — the draft listing needs a valid GS1-issued product identifier to exist at all. FNSKU comes later in the sequence: it's generated inside Seller Central only once the listing is live, then must be downloaded and forwarded to the supplier so they can label every physical unit with it before the shipment leaves the factory.