By default, Odoo sets a minimum quantity of 0 for all items in WH/Stock. When the forecasted quantity goes below the minimum of 0, Odoo detects the need and shows it under the Replenishment menu. A user can then turn this "Manual" reordering rule to a reordering rule with an "Auto" trigger. 

If I have 3 units on hand, a sales order for 20 and no reordering rule, the Replenishment menu shows I need to order 17.

A user can create a reordering rule when creating products and the configurations remain the same. In the example below, I am assuming:

• One stock location (WH/Stock)
• One-step receipt

Follow these steps:

1. Create a reordering rule for the product:
2. 1. Min = 0
   2. Max = 0
   3. Multiple Quantity = 0
   4. Trigger = Auto
   5. 
      
3. Make sure that the product is has a vendor listed and the "Buy" route
4. 1. 

When the sales order is placed, Odoo creates a purchase order for 15 pieces.