With override enabled, the primary unit with the highest device priority will always become the primary unit. Whenever an event occurs that may affect primary unit selection, the cluster negotiates. For example, when override is enabled a cluster renegotiates when you change the device priority of any cluster unit or when you add a new unit to a cluster. Override and primary unit selection Enabling override changes the order of primary unit selection. As shown below, if override is enabled, primary unit selection considers device priority before age and serial number. This means that if you set the device priority higher on one cluster unit, with override enabled this cluster unit becomes the primary unit even if its age and serial number are lower than other cluster units. https://docs.fortinet.com/document/fortigate/6.0.0/handbook/123439/primary-unit-selection-with-override-enabled