:The recovery catalog is optional. If a The controlfile(s) of the target database are the sole repository of metadata if a recovery catalog is not used. A recovery catalog is a set of tables and views used by RMAN to store information about databases. RMAN uses this data to manage the BACKUP, RESTORE, and RECOVERY of databases.

Connect to the recovery catalog database either through the RMAN command-line CATALOG argument or CONNECT CATALOG. The recovery catalog database must be open.

CREATE CATALOG [TABLESPACE 'tablespace_name']
 Use to create a schema for the recovery catalog. Typically, created in a separate database. The catalog is created in the default tablespace of the recovery catalog owner.
The recovery catalog owner must be granted the RECOVERY_CATALOG_OWNER role as well as space privileges on the tablespace where the recovery catalog tables will reside.

DROP CATALOG
 Deletes all metadata from the recovery catalog. Enter the command twice to confirm that the schema is to be dropped.

RESYNC CATALOG [FROM CONTROLFILECOPY 'filename']
Performs a full resynchronization of the recovery catalog. Resynchronizations can be full or partial. When full, RMAN updates all changed records for the physical schema: datafiles, tablespaces, redo threads, and online redo logs. If the database is open, RMAN also obtains data about rollback segments.
When partial, RMAN reads the current control file to update data, but does not resynchronize metadata about the physical schema or rollback segments.
Typically, RMAN automatically executes a full or partial resynchronization as needed RMAN commands are executed. A resync is not performed automatically when:
The recovery catalog is unavailable.
When a log switch occurs or when an online redo log is archived for a database in archive log mode.
When changes have been made to the physical structure of the target database such as adding or dropping a tablespace.

UPGRADE CATALOG [TABLESPACE 'tablespace_name']
To upgrade the recovery catalog schema from an older version to the version required by the RMAN executable.