Email Notification to Buyer in Back to Back Order process?

Could Someone let me know how to enable email notifications to buyers whenever there is a change made in back to back orders?Say if someone updates quantity - increase/decrease,  what setups are required to enable the email notification so that the buyer can change the quantity on PO accordingly.

Here is snippet from one of the Oracle white paper.

"  If the sales order line is cancelled or the quantity is reduced, then the reservation is reduced and a notification is automatically sent to the buyer explaining that there is now a PO outstanding for a higher quantity than what is needed for the sales order."