Multi-pack barcodes allow you to pre-pack a set quantity of the item and assign a unique barcode with an associated quantity to the pack. The pre-counted pack is labeled with the barcode you define. Scanning the multi-pack barcode will have the same effect as scanning a quantity of items equal to the quantity assigned to the pack.

Consider the following use case: The customer orders 105 of a given item. For efficiency, your items are pre-counted in packs of 25. To use the multi-pack barcode feature you would label each pack with a multi-pack barcode which has been assigned the value 25. 

The individual items not in packs are also barcoded with a regular (non-multi-pack) barcode. To scan 105 of the item, the packer would begin by scanning 4 of the multi-packs. With each scan, 25 of the item would be marked scanned in the order. The remaining 5 individual items would then be scanned to complete scanning for the item.

You can quickly add multipack barcodes right in the GroovePacker interface or using a CSV. 

If the box you use for a case of your items has a barcode from the box manufacturer you can often use this as a multi-pack barcode. Just add the case qty as the multi-pack barcode qty and you're good to go.

How is this different than a Kit or Case containing many of the same item?

A kit could actually be a case of 12, 25 etc. but a kit should be used when the quantity of the order line item reflects the number of kits rather than the number of items inside the kit. So if the customer orders 1 case and should receive 25 items a kit should be used. If the customer orders 25 and 25 items will be scanned (in a pack or in a combination of packs and single items) then the multi-pack barcode should be used.

Another difference is that a kit can contain different skus whereas a multi-pack will always be 2 or more of the same sku.