The Package SKU field in KlickTrack refers the to the scannable barcode on a product. This barcode can be associated with a product directly through inbound manifesting product or can be added/edited via the inventory detail page. This barcode is used for the register functions such as scanning and product look up.
The Purchase SKU on an item refers to the traceability (Global) barcode that identifies the product for state traceability. This barcode is required to manifest a product through a state traceability system.