Hi
Crosschecks update outdated RMAN repository information about backups whose repository records do not match their physical status. For example, if a user removes archived logs from disk with an operating system command, the repository still indicates that the logs are on disk, when in fact they are not.
If the backup is on disk, then the CROSSCHECK command determines whether the header of the file is valid. If the backup is on tape, then the command simply checks that the backup exists. The possible status values for backups are AVAILABLE, UNAVAILABLE, and EXPIRED.

you can list the backup details using the commands
list backup;
list backup summary;

or by quering
V$BACKUP_FILES
RC_DATAFILE_COPY or RC_ARCHIVED_LOG.

f the backup is no longer available, then RMAN marks it as EXPIRED.
gopu