Reordering rules and make to order do both allow to automatically replenish products when necessary. The difference between those tools is how they are triggered.
Reordering rules are used to ensure that you always have a minimum amount of a product in stock in order to manufacture your products and/or answer to your customer needs. When the stock level of a product reaches its minimum the system will automatically generate a procurement with the needed quantity to reach the maximum stock level.
The Make to Order route will trigger a procurement (purchase order or manufacturing order) with the necessary quantity to fulfill a product request (coming from a sale order or a manufacturing order). The system will not check the quantity on hand for the corresponding product.
The Reordering Rules configuration is available through the menu Inventory --> Master Data --> Reordering Rules. There, click on Create to set minimum and maximum stock values for a given product.
When the forecasted stock goes below the Minimum Quantity specified in this field, Odoo generates a procurement to bring the forecasted quantity to the Maximum Quantity. The Quantity Multiple is used to round the procurement quantity up to this multiple. If it is 0, the exact quantity will be used.
The Lead time is the number of days after the order point is triggered to receive the products or to order them to the vendor.
In case you work with multi-warehouses and/or multi-locations, you will be able to specify different reordering rules for the same product in each location.
For the reordering rules to be triggered, on the corresponding product, a route should be specified. In case you manufacture your products, make sure to select the route Manufacture and create a Bill of Material for the product. In case you purchase your products, make sure to select the route Buy and add a Vendor Pricelist.
Don't forget to select the product type storable in the product form. A consumable can not be stocked and won’t trigger reordering rules.
Make to Order
The Make To Order configuration is available on your product form through your Inventory module --> Master Data --> Products (or in any other module where products are available).
On the product form, in the Inventory tab, you can set a route. You can choose Make To Order and another route, either Buy or Manufacture.
Again, if the Buy route is selected, make sure to set a Vendor pricelist in the Purchase tab. In case you have set a Manufacture route, make sure you have a BoM for this product.