Viewing Associated Order Lines allows you to view where an item on a Purchase Order originated from.

On Non stock inventory tab, if you double click on the quantity or Stock # fields it will bring up the following which is a listing of all customer order line items linked to the purchase order line item:

On this screen you can see the order # the line item is Associated with,  you can double click on the order number field to open the actual order (See Maintain an Order) if needed.

On Standard Stock Inventory Tab if you double click on the part # it will bring up the maintain product screen (See Maintain Products).