Chapter 15 Unified Threat Management for FortiOS 5.0 : Intrusion protection : Configure IPS options : Configuring the session count accuracy
  
Configuring the session count accuracy
The IPS engine can keep track of the number of open session in two ways. An accurate count uses more resources than a less accurate heuristic count.
config ips global
set session-limit-mode {accurate | heuristic}
end
The default is heuristic.