Chapter 15 Unified Threat Management for FortiOS 5.0 : Intrusion protection : IPS concepts : Signature-based defense : Policies
  
Policies
To use an IPS sensor, you must select it in a security policy or an interface policy. An IPS sensor that it not selected in a policy will have no effect on network traffic.
IPS is most often configured as part of a security policy. Unless stated otherwise, discussion of IPS sensor use will be in regards to firewall policies in this document.