Update: Stock users will now follow a different process for marking items received. If you are a Stock user, please use this article instead: Marking Items Received with Stock.

After you have created and sent your Purchase Order, you can use the list in the Purchasing area to keep track of which items have arrived.

To do this, open your project and click Purchasing, then select the items that have arrived and click "Mark Items Received". 

This will update the list with the date the items were received.