When using "format sbt_tape", RMAN does not know where it's backuppieces are going. It's just giving them to the storage manager (i.e. NetBackup). In this case, you can use disk staging units.
If you prefer to have RMAN backup to disk directly, ("format disk") then the RMAN disk images are backed-up as standard files. In this case I'd recommend putting the RMAN backup script in a NetBackup bpstart_nofiy script, and archive them rather than back-up them.
Hope this information helps.
Brice Avila