In the Purchasing Data Tab, the "Purchasing UoM" refers to the units that the vendor uses in the invoice. "Items per Purchase Unit" refers to how many inventory items are in each Purchasing UoM. So you can enter an invoice for 1 Box of Keg Caps (Purchasing UoM), and this results in 1000 keg caps (the Items per Purchase Unit) showing up in my inventory.
Now we get to "Packaging UoM (Purchasing)" which refers to the standard number of Purchasing Units I have to buy at once. For example, you might always buy a pallet (Packing UoM) of 5 boxes (Quantity per Packaging Unit) of keg caps at once. However, the packaging UoM is simply a reference field for use in reporting and making purchasing decisions. It does not have any affect any of the marketing documents for purchasing; these only look at the Purchasing UoM.