Chapter 19 Virtual Domains : Virtual Domains in Transparent mode : Configuring VDOMs in Transparent mode : Creating security policies
  
Creating security policies
Security policies permit communication between the FortiGate unit’s network interfaces based on source and destination IP addresses. Typically you will also limit communication to desired times and services for additional security.
In Transparent mode, the FortiGate unit performs antivirus and antispam scanning on each packet as it passes through the unit. You need security policies to permit packets to pass from the VLAN interface where they enter the unit to the VLAN interface where they exit the unit. If there are no security policies configured, no packets will be allowed to pass from one interface to another. For more information, see the FortiGate Administration Guide, or FortiGate Fundamentals Guide.