Currently, an alert can be issued when inventory fallen below a threshold. But then what? How does purchasing know how much to buy?

Adding a e.g. "Recommended stock level" or "Upper stock level" would allow the current stock to be subtracted from this number, giving the number of items that should be purchased. Integrating with a report would allow the number of items required to be added up across sites very easily, including if low thresholds have not been reach at other sites, but, seeing as the order is being placed, the difference between the Upper and Remaining levels could be included in the purchase order, reducing shipping and admin costs.