Explanation:
Before forwarding a client request to a database server, the listener determines if a database service handlers are available through a service registration. The background process is responsible for providing this information to the listener. This enables the listener to direct the client requests to the appropriate service.
Reference:
Oracle 9i Net Services Administrator’s Guide
Oracle 9i Net Services Reference Guide
Oracle 9i Recovery Manager Reference
Oracle 9i Backup and Recovery Concepts

---