Retention periods can be different for full (all object) and incremental (changed objects) backups. During a recovery, BRMS uses the latest full backup and any subsequent incremental backups to restore that control group or backup item to its most current status.
BRMS allows you to specify the type of incremental backup that you want to use. A cumulative (*CUML) type of incremental backup specifies a backup that includes all changed objects and new objects since the last full backup. An incremental (*INCR) backup type specifies that incremental backups include all changed objects and new objects since the last incremental or full backup. You can specify the type of incremental backup (*CUML or *INCR) in the backup policy. You also can specify the type individually in each backup control group.
It is important that the retention period for incremental backups be synchronized between each other as well as with full backups. This assures full system recovery in case of system failure. BRMS uses its media content information, and media inventory, to determine the most important information to recover. BRMS bases its recovery process on the media and media information that you keep.