What factor make a device Active and another Standby, and when Active become standby does it become

With system fail-safe, the BIG-IP system monitors various hardware components, as well as the heartbeat of various system services. You can configure the system to initiate failover whenever it detects a heartbeat failure.

With gateway fail-safe, the BIG-IP system monitors traffic between an active BIG-IP system in a device group and a pool containing a gateway router. You can configure the system to initiate failover whenever some number of gateway routers in a pool of routers becomes unreachable.

With VLAN fail-safe, the BIG-IP system monitors network traffic going through a specified VLAN. You can configure the system to initiate failover whenever the system detects a loss of traffic on the VLAN and the fail-safe timeout period has elapsed.

With an HA group, the BIG-IP system monitors trunk, pool, or cluster health to create an HA health score for a device. You can configure the system to initiate failover whenever the health score falls below configurable levels.

When you enable auto-failback, a traffic group that has failed over to another device fails back to a preferred device when that device is available. If you do not enable auto-failback for a traffic group, and the traffic group fails over to another device, the traffic group remains active on that device until that device becomes unavailable.