When you configure a buy route, you can purchase products in one of two ways. One of the ways to purchase this product would be to create a purchase order and add the product to the purchase order manually. This is the typical manual purchasing system, where a purchase agent perhaps uses other events outside Odoo or examines reports in Odoo to create purchase orders.

In addition to creating manual purchase orders, you can also create re-ordering rules that will automatically create draft purchase orders when the stock of the product dips below a set minimum. This method works well on products that are ordered frequently, and frees your purchasing managers from having to manually create purchase orders for some or even most of your inventory.