A、 It must be used because all RMAN-related backup and recovery metadata information is contained in it.

B、 It provides a convenient， optional， repository of backup- and recovery-related metadata.

C、 It provides the ability to store RMAN scripts for global use by any database that has access to the repository.

D、 It provides a means of storing all RMAN backup sets physically in an Oracle database server. 

E、 It provides the ability to store backup records for more than a year.