By default, Amazon has a setting turned on in a Seller's account to use the manufacturer barcode for their items. This is also known as Stickerless Commingling
To turn off Stickerless, Commingled Inventory if you would like:
Seller Central->Settings->Fulfillment by Amazon
Scroll down to FBA Product Barcode Preference
Click the Edit button to the far right
Select Amazon barcode and click Update
Note: Any changes you make to this setting immediately takes effect in the same batch.
If your settings are NO_LABEL when you add your first item to a batch, you will be notified and prompted to change the setting if necessary.
If you choose to update your preference, you will be taken to the barcode settings page in Seller Central. Once you change the setting and return to InventoryLab, a new MSKU will be created once you choose 'Add to Batch'. If you continue as commingled, your item will add to your batch as NO_LABEL.
For more information on this topic, feel free to check out Amazon's Help Page.