The basic difference between the Routes BUY and Make To Order are as follows :-

1. The option BUY is primarily used when you wish to maintain physical inventory/stock of a product. Here you will always maintain a certain stock of products so that the same can be delivered to your customer.

   For E.g. You want to deliver I- phones from your inventory only and for that you will always maintain a certain inventory/stock of I-phones and in that case only you will use the route BUY. The stock of products/goods will decrease w.r.t how many orders are delivered.

2. The option Make to Order is primarily used when you don't wish to maintain a stock/inventory of products. It has nothing to do with inventory(even if there is certain quantity of product in inventory). Here as soon as your customer places an order you will procure that product from your supplier and deliver it to your customer

   For E.g. If you sell I-Phones and have a supply chain route as Make To Order. Hence,as soon as your customer places an order a procurement will be generated which will in return generates a Purchase Order and you will either deliver the product to the customer though your supplier directly or through Drop Shipping.