What feature can be changed after Sync?

When you perform a config sync from A to B, they will end up with the same configuration. If you then make changes to either box, they'll show that another config sync is required.

You can also set the device-group to 'auto sync', which does it for you automatically when a change is detected.

Some customers prefer to use auto-sync, so that their devices always have the same config, and others prefer to make changes, test them, and leave the standby device untouched until they're happy with the changes, so that if they need to revert, they can just failover to the standby.