Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : FGCP HA terminology : Subordinate unit
  
Subordinate unit
Also called the subordinate cluster unit, each cluster contains one or more cluster units that are not functioning as the primary unit. Subordinate units are always waiting to become the primary unit. If a subordinate unit does not receive hello packets from the primary unit, it attempts to become the primary unit.
In an active-active cluster, subordinate units keep track of cluster connections, keep their configurations and routing tables synchronized with the primary unit, and process network traffic assigned to them by the primary unit. In an active-passive cluster, subordinate units do not process network traffic. However, active-passive subordinate units do keep track of cluster connections and do keep their configurations and routing tables synchronized with the primary unit.
The FortiGate firmware uses the terms slave and subsidiary unit to refer to a subordinate unit.