FNSKU
Also known as: Fulfillment Network SKU
The Amazon-issued barcode that uniquely identifies your specific units of a SKU within Amazon's fulfillment network.
FNSKU (Fulfillment Network SKU) is the barcode Amazon assigns to your units of a particular SKU when you list it for FBA. Two different sellers offering the same ASIN have different FNSKUs — Amazon uses the FNSKU to keep each seller's inventory separate within the fulfillment center, even if both sellers' physical units are stored on the same shelf.
The FNSKU prints onto the product label that goes on every unit before you ship it to FBA (unless you use Amazon's Stickerless, Commingled inventory program — which most sellers should avoid because it pools your inventory with everyone else's and can cause reimbursement headaches when commingled units come back damaged or counterfeit).
When you create an inbound shipment in Seller Central or via SellerVault's shipment wizard, the FNSKU is what gets printed onto the prep labels. If a label is wrong, Amazon will receive your units against the wrong FNSKU and you'll spend hours opening cases to fix it. Always verify FNSKU labels before shipping.