This parameter defines a special retention time for RMAN backups. It should only be used for special backups (monthly or yearly) that are not part of the standard retention strategy.
rman_keep = <days>
Default: 0 - standard retention period from the RMAN configuration
<days>: retention period in days of an RMAN backup in the control file
See also:
RMAN Backup with an External Backup Library
Oracle documentation