FBA labeling is the process of attaching a unique product identifier (such as a SKU) with its barcode onto the product.
The FBA labels need to be attached to each unit or to the outside packaging of the unit so that the entire FBA label is visible. You need to ensure that each unit that is shipped to Amazon fulfillment centers has a FBA label on its packaging that complies with these rules, including a scannable barcode containing the correct Amazon identifier.
If any unit is shrink-wrapped or in a protective case, you will need to ensure that the barcode is scannable without needing to remove the shrink wrap or case. The FBA labels must also cover the original barcode.