Announcement reposted from Amazon Europe:
We’ve improved the barcode scanning process at our fulfilment centres to enable faster processing with automation. If your FBA products use FNSKU labels that start with an X, make sure that you cover all secondary barcodes (such as EAN or UPC) so that our system can process your items quickly and accurately.
If you use FNSKU labels but your product has other barcodes visible, our system may scan one of the other barcodes instead of the FNSKU, which can cause processing delays and potential losses.
For more information, go to FBA product barcode requirements and Use Amazon barcodes to track inventory.
You can also review our FBA Labelling Guide.