Explanation
References: B - Support for Oracle Cloud Infrastructure, other Cloud provider endpoints (AWS, Azure) and private assets, including Cloud, CDNs and Data CentersC - Customers may purchase Oracle Cloud Infrastructure Private Pool and Vanity Nameserver to have their Domain Names and Zones under a private IP pool with dedicated nameservers to segregate from those of other customers in order to reduce the risk of external issues affecting their websites.
https://www.oracle.com/cloud/networking/dns-faq.html