"Item Package Quantity" refers to the number of subunits contained in the unit.
Example: For a single 3-pack of T-shirts, where the T-shirts are packaged all together with one barcode, enter "1" as the quantity. If the item for sale is two 3-packs of T-shirts, enter "2."
Jet and Walmart have the same definition for this field. You can read more here under #4: https://partner.jet.com/catalog-guide