This column shows (by default) the quantity per package that was entered for the respective part in the order. The quantity per package is registered under the Shipping info tab in the Update Part procedure in either the Inventory or the Sales module.
You can change Quantity per package before printing the transport label, e g. if the physical quantity is larger or smaller (numerous) than the entered quantity per package.
If this field is left empty, the quantity per package will not be printed on that transport label at all.