You can specify different criteria for REPORT NEED BACKUP, using one of the following forms of the command:

REPORT NEED BACKUP RECOVERY WINDOW OF n DAYS

Displays objects requiring backup to satisfy a recovery window-based retention policy.
REPORT NEED BACKUP REDUNDANCY n

Displays objects requiring backup to satisfy a redundancy-based retention policy.
REPORT NEED BACKUP DAYS n

Displays files that require more than n days' worth of archived redo log files for recovery.
REPORT NEED BACKUP INCREMENTAL n

Displays files that require application of more than n incremental backups for recovery.

http://docs.oracle.com/cd/B28359_01/backup.111/b28270/rcmreprt.htm#BRADV89595

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Using RMAN REPORT NEED BACKUP with Different Retention Policies