Explanation/Reference: https://docs.cloud.oracle.com/en-us/iaas/Content/Database/Tasks/exausingdataguard.htm

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.

A mount target is an NFS endpoint that lives in a VCN subnet of your choice and provides network access for file systems. The mount target provides the IP address or DNS name that is used together with a unique export path to mount the file system. A single mount target can export many file systems. Typically, you create your first mount target and export when you create your first file system. The mount target maintains an export set which contains all of the exports for its associated file systems.
Limitations and Considerations
Each availability domain is limited to two mount targets by default. However, you can export up to 100 file systems through each mount target.
See Service Limits for a list of applicable limits and instructions for requesting a limit increase.
Each mount target requires three internal IP addresses in the subnet to function. Two of the IP addresses are used during mount target creation. The third IP address must remain available for the mount target to use for high availability failover.
The File Storage service doesn't "reserve" the third IP address required for high availability failover. Use care when designing your subnets and file systems to ensure that sufficient IP addresses remain available for your mount targets.