Some suppliers may ask you to have their SKU number of the purchase order, so they can process them easier.
When you are creating a new product or editing an existing one, you will see a 'SKU (supplier)' field in the fulfillment section.
If you are editing a product, click on the edit button to enable changing and type in the stock keeping unit. Save the document and exit this window.
You can create a supplier s SKU for every product that you have in your database. Now, when you create a PO for this product, you will get the suppliers SKU number on the print preview.
If you add a product without your suppliers SKU number assigned to it, it will just show you your own SKU number on that product.