If the unit has an UPC barcode, the unit is commingled (also called "stickerless" inventory). Commingling means Amazon can use one seller's inventory to fulfill another's order - the units are never stored in the same bin to allow tracking origin.
If the unit has an Amazon seller-specific FNSKU barcode, the unit is not commingled.
The used barcode type can be configured by the seller.
The barcode configuration option does not mention commingling at all (but help pages do), so I can understand some sellers getting confused on whether they are affected by commingling or not.
