Single floating IP address (also known as a floating IP or shared IP) is supported only in an active/passive HA pair. In active/active HA, both firewalls are forwarding traffic simultaneously and thus do not share a single floating IP.
In active/passive HA, a single floating IP address is used for seamless failover. Active/active HA requires separate IP addresses and does not support a single floating IP.
This simplifies failover in active/passive deployments by using a single shared IP that moves to the active peer upon failover.