The FBA SKU (FNSKU) is an Amazon product identifier for products that are fulfilled by Amazon. The FNSKU identifies the product as yours.You need an FNSKU in order to create FBA Inbound Shipments.
To get the FNSKU, set the product as Fulfilled by Amazon, and then launch it to Amazon. After a short period of time, Amazon will assign an FNSKU to the product. Click Get FNSKU on the Action Menu of the Amazon Properties page.
You can also get the FNSKU for multiple products when creating an FBA Inbound Shipment. Select the products missing the FNSKU, and click Get FNSKU on the Action Menu.