Explanation
You can permanently terminate (delete) instances that you no longer need.By default, the instance's boot volume is deleted when you terminate the instance, however you can preserve the boot volume associated with the instance, so that you can attach it to a different instance as a data volume, or use it to launch a new instance.
You can use a boot volume backup to create an instance or you can attach it to another instance as a data volume. However before you can use a boot volume backup, you need to restore it to a boot volume.