FNSKU

When you send inventory into Fulfilment by Amazon, Amazon needs a way to know which physical units belong to which seller and which product. The FNSKU is that link. It is a barcode, usually starting with the letter X, that Amazon generates for each of your products when you set them up for FBA. You print it onto a label and apply it to each unit before shipping it to the fulfilment centre.

The FNSKU sits at the warehouse-operations layer of Amazon's identifier stack. The GTIN says what the product universally is; the ASIN identifies the product page on the marketplace; the FNSKU identifies a physical unit of your stock inside Amazon's network. All three can refer to the same product but answer different questions.

By default, Amazon can use either manufacturer barcodes or Amazon FNSKU labels to manage FBA stock. If you let Amazon track items by the manufacturer barcode, your units may be commingled — pooled with identical units from other sellers — and a buyer who orders from you might be shipped a different seller's stock. Applying your own FNSKU label prevents this: it ensures the exact units you sent are the ones fulfilled against your orders.

For most sellers, especially private-label brands, FNSKU labelling is strongly preferred. It protects you from being held responsible for another seller's counterfeit or damaged goods, and it keeps your inventory accounting clean. Amazon offers a paid label service, or you can print and apply the FNSKU labels yourself or via a prep centre.