The docs may help you. It describes how listening and traffic forwarding work.
" About operation mode & server policy behavior"
http://docs.fortinet.com/fweb/html/fortiweb-admin/Policy_29_2.html
" Before you can create a server policy, you must first configure a virtual server that defines the network interface or bridge and IP address where traffic destined for an individual web server or server farm will arrive. ... simply defines the listening network interface."
http://docs.fortinet.com/fweb/html/fortiweb-admin/server_objects_22_10.html
" Server farms define a group of physical and domain servers (web servers) among which the FortiWeb appliance will distribute connections, or where the connections will pass through to, depending on the FortiWeb appliance’s operating mode. (Reverse proxy mode actively distributes connections; offline protection and both transparent modes do not.)"
http://docs.fortinet.com/fweb/html/fortiweb-admin/server_objects_22_6.html