Explanation
A mount target is an NFS endpoint that lives in a VCN subnet of your choice and provides network access for file systems. The mounttarget 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
Eachavailability 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.

Explanation
When Autonomous Databas instance is stopped,
CPU billing is halted based on full-hour cycles of usage
Billing for storage continues as long as the service instance exists.
and When AutonomousDatabase instance is started, the CPU billing is initiated

Explanation
https://docs.cloud.oracle.com/en-us/iaas/Content/Resources/Assets/whitepapers/deploying-custom-os-images.pdf A template of a virtual hard drive that determines the operating system and other software for an instance. For details about Oracle Cloud Infrastructure platform images, see Oracle-Provided Images. You can also launch instances from:
Trusted third-party images published by Oracle partners from the Partner Image catalog. For more information about partner images, see Overview of Marketplace and Working with Listings.
Pre-built Oracle enterprise images and solutions enabled for Oracle Cloud Infrastructure Custom images, including bring your own image scenarios.
Boot Volumes.
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