Chapter 22 WAN Optimization, Web Cache, Explicit Proxy, and WCCP for FortiOS 5.0 : The FortiGate explicit web proxy : Web Proxy firewall services and service groups
  
Web Proxy firewall services and service groups
Configure web proxy services by selecting Explicit Proxy when configuring a service. Web proxy services can only be selected in a security policy when web-proxy is selected as the source interface.
Web proxy services are similar to standard firewall services. You can configure web proxy services to define one or more protocols and port numbers that are associated with each web proxy service. Web proxy services can also be grouped into web proxy service groups.