You host VPS servers or another service using OVH dedicated servers and, you have IP blocks from OVH. Your customer is assigned an IP and, this is listed in the customers WHMCS profile. They have an IP in the Dedicated IP field and more IPs in the Assigned IP field. Your customers VPS server is the target of a DDoS attack aimed at the servers main IP. OVH send you a warning to the email address you have registered with them saying mitigation has been activated.
The OVH Notification module scans your email account and matches the IP in the OVH DDoS warning email to the IP in your customers WHMCS profile. At this point, the module triggers the DDoS warning email and sends this to your customer. Once the attack has ended OVH will send another email saying mitigation has been disabled and again, the module will send the DDoS attack ended email to the customer.
All you need to know about the WHMCS OVH Notification Module
The answers on most common questions are described bellow.