Oracle XML DB
Oracle XML DB adds native XML support to your DB instance. With XML DB, you can store and retrieve structured or unstructured XML and relational data. The XML DB protocol server isn't supported on RDS for Oracle.
XML DB is preinstalled on Oracle Database 12c and higher. Thus, you don't need to use an option group to explicitly install XML DB as an additional feature.
To learn how to configure and use XML DB, see Oracle XML DB
Developer's Guide
