A SKU (stock keeping unit) is the identifier you assign to an offer when you create it in Seller Central. Amazon calls it the seller SKU or merchant SKU. Unlike the ASIN, which belongs to Amazon's catalog and is shared by every seller of a product, the SKU is yours: you choose it, it can be up to 40 characters, and it is how Amazon refers to your offer in inventory, orders, shipments and most downloadable reports. For FBA, Amazon also generates an FNSKU, the barcode that ties physical units to your SKU in its warehouses.
Why it matters
One ASIN can have several of your SKUs behind it: an FBA offer and a merchant-fulfilled offer, or an old SKU and a replacement created after a relisting. That is legitimate and sometimes useful, but it also causes confusion, because sales history, inventory and advertising are reported by SKU in some places and by ASIN in others. Stranded inventory, where FBA stock exists but no active SKU offers it, is a SKU-level problem that looks like a stock-out on the listing.
A consistent SKU scheme also makes bulk operations safe. Flat-file uploads, price changes and inventory feeds all key on SKU, so a scheme that encodes brand, product, variant and fulfillment method lets a person read a bulk file and catch errors before upload.
What moves it
- Naming discipline. SKUs cannot be renamed once created; a new one must be made and the old one closed.
- Fulfillment channel. Switching a SKU between FBA and FBM changes how it is treated in reports and can strand inventory during the switch.
- Duplicates. Two SKUs on one ASIN with the same fulfillment method compete with each other for the Buy Box.
How we use it
In an onboarding audit we map every SKU to its ASIN and flag duplicates, closed SKUs that still hold FBA stock, and SKUs whose costs are missing from the profit model. Our bulk changes are prepared and validated by SKU before upload, and the change log in your report records the SKU as well as the ASIN so every edit can be traced.