The FNSKU is a value unique to Amazon that used to track FBA inventory. Amazon sends the FNSKU value to Zentail, where it is stored in Zentail's database. It is not visible in Zentail's UI. However, since Zentail knows what the FNSKU is we're able to sync up to the correct value used on Amazon, and use that as the barcode for product labeling where appropriate, for FBA inbound shipments.