Setting up a Re-Order or Re-Stock level allows for the system to do just that.  When an inventory level for a certain product reaches a certain level, it will create a purchase order for you in order to ensure that the inventory is replenished when you need it.  

How to set a Re-Order/Re-Stock Level

To start, we go to our "Catalog">"Manage Products" tab of the Store Operations menu.  To ensure that you can add a Re-Order/Re-Stock level to any product you are looking at, ensure that you have visibility set to "Show All". This is found by clicking "More Options" and "Visibility Show All".  This will allow you to see all of the Simple Variants of your products. Afterwards, please proceed to edit an item.  In the edit item screen, click on the Inventory tab to get to where we need to be, as shown below.

Here you will see a list of your stores.  In order to set a re-order/re-stock quantity for the item you are editing, it is required that you click on the desired store you want to set it for first before clicking the Set Re-Order Qty button. After ensuring a store is selected, click the Set Re-Order Qty button and you will be shown a screen as depicted below.

Here you can fill in the stock level in which you want the re-order to take place and the amount of the item you want to replenish.

After setting the re-order level, in order to make a Purchase Order for items that have reached your re-order threshold, please proceed to making a new Purchase Order.  From here, after you select a store and supplier, you can navigate to the Item Details Menu and hit "Add Items to PO" from the Action Menu>Tools as shown below.

Hitting this will bring up the following menu and from there it will add any items from that supplier that need to be reordered.

Hit save and you have completed the process!