All products sent to Amazon distribution centers for fulfillment via FBA are assigned a unique FNSKU when the seller creates an Inbound Shipment. It is displayed on a label that the seller must print out and attach to each item before shipping to the fulfillment center. Alternatively, Amazon can print and attach these labels themselves, if the seller uses the paid FBA Label Service. If Amazon receives any product units without a label, then it will charge for the Unplanned Prep Service.