Difference between Procurement Contract and Purchase Agreement in Oracle Fusion

Content

What is the exact difference between a procurement contract (Enterprise Contract) and a purchase agreement (BPA)? 

From functional point of view, we can add items/service, terms and conditions etc. to both document. Both documents can be used to create a purchase order as well. I need to understand for which use cases we can use a contract for for which use case we use a purchase agreement.

Thanks

Version

11.13.21.04.0