This parameter defines additional parameters that RMAN passes to an external backup library, that is, backup_dev_type = rman_util|rman_disk|rman_stage.
Ask the vendor of the external backup library if it requires you to set additional parameters.
Syntax
rman_parms = "<string>"
Default value: none
Example
rman_parms = "BLKSIZE=65536 ENV=(BACKUP_SERVER=HOSTNAME)"
See also:
RMAN Backup with an External Backup Library
Oracle documentation