There are times where data might not be formatted correctly or wrong data provided that will cause a purchase to be returned from the supplier. The supplier will often return an message that will give you an indication of what data is incorrect. This could be a missing part number, wrong contact information, missing address information for example. The purchase request form will allow you to see the errors. You can see and filter on the orders that have been return with an error and then address them with the owner of the purchase order.
Here is a quick overview
Refer to this post for an overview of the cXML Purchase Order Send.