Chapter 17 Traffic Shaping for FortiOS 5.0 : The purpose of traffic shaping : Bandwidth guarantee, limit, and priority interactions : FortiGate traffic
  
FortiGate traffic
Administrative access to the FortiGate through HTTPS or SSH, or IPsec tunnel negotiations, security policies do not apply, and therefore FortiGate units do not apply traffic shaping. Such traffic also uses the highest priority queue, queue 0. In other words:
packet priority = 0
Exceptions to this rule include traffic types that are connections related to a session governed by a security policy.
For example, if you have enabled scanning by FortiGuard antivirus, traffic from the sender technically terminates at the FortiGate proxy that scans that traffic type; the FortiGate unit initiates a second connection that transmits scanned content to its destination. Because the second connection’s traffic is technically originating from the FortiGate proxy and therefore the FortiGate unit itself, it uses the highest priority queue, queue 0. However, this connection is logically associated with through traffic, and is therefore subject to possible bandwidth enforcement and guarantees in its governing security policy. In this way, it behaves partly like other through traffic.
See also 
Through traffic
Quality of Service
Traffic policing
Calculation and regulation of packet rates