Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : FGCP HA terminology : Session failover
  
Session failover
Session failover means that a cluster maintains active network sessions after a device or link failover. FortiGate HA does not support session failover by default. To enable session failover you must change the HA configuration to select Enable Session Pick-up. See also “Device failover, link failover, and session failover”.