Chapter 15 Unified Threat Management for FortiOS 5.0 : Security Profiles overview : Traffic inspection : Application control : Application control recommendations
  
Application control recommendations
Some applications behave in an unusual manner in regards to application control. For more information, see “Application considerations”.
By default, application control allows the applications not specified in the application control list. For high security networks, you may want to change this behavior so that only the explicitly allowed applications are permitted.