Reference | Defining static IP addresses

If not explicitly set, the private IP address will change if you deprovision an instance and reprovision it. You can specify the IP associated with this instance to avoid connectivity issues or to update your DNS entry pointing to this IP address. This IP address must be part of the subnet where your instance is provisioned. In addition, you’ll need to ensure it is available, so the provisioning doesn’t fail.

For Elastic IPs you will need to specify the ID of an EIP you have previously created.