A physical IP address is unique to an interface on a hardware device on your network.
When you have multiple hardware devices in the same HA group, the logical IP address typically points to the active hardware device's physical IP address. When you have an HA failover, it will point to the new active device.
Under F5, floating interface carries an additional meaning. It's used for communicating with the back-end servers. Therefore, it is NOT needed on the subnet that the VS are on unless you have back-end servers on the same subnet.