There are cases when you want a purchase order to be created automatically when the stock quantity of some product reaches the minimum level. For such case, you can create a reordering rule for that product where you will specify the minimum stock quantity that the product can have in the system. Also, you can configure what is the maximum stock quantity that the product can have and what is the lowest number of items that can be ordered at once.
For example, if you have a product Tablet A for which you want to create a new purchase order always when there is only one item in stock, but also you don’t want more than 10 items in stock, you can define a reordering rule where as Minimum Quantity will be set 1 and Maximum Quantity 10.
More details how to create a Reordering Rule you can find here