As far as I understand it, that just means that sellers have the option to opt in/out of the commingling program (Settings => Fulfillment by Amazon => FBA Prod

As far as I understand it, that just means that sellers have the option to opt in/out of the commingling program (Settings => Fulfillment by Amazon => FBA Product Barcode Preference). Yes, there is a cost for the seller if opting out - they need to apply their own barcodes to the products in that case.