If you intend to configure your Oracle backup using both NetBackup for Oracle and RMAN "backup proxy" command, RMAN and Snapshot Client feature are required. Snapshot Client is responsible for takeing snapshot.
Sequence of operation is detailed in docs below.
How does NetBackup for Oracle with Snapshot Client work?
- NetBackup for Oracle Administrator's Guide, page 143
http://www.symantec.com/docs/DOC3672
NetBackup Snapshot Client Administrator's Guide
http://www.symantec.com/docs/DOC3661
If backup is configured without NetBackup for Oracle(Oracle Agent), RMAN is not used.
Also, If backup is configured without snapshot client, NetBackup is not responsible for taking snaphosts.