Check oracle rman doc for catalog command

Use the CATALOG command to do the following:

Add backup pieces and image copies on disk to the RMAN repository.

Record a datafile copy as a level 0 incremental backup in the RMAN repository, which enables you to use it as part of an incremental backup strategy.

Record the existence of the last user-managed datafile copies made after the final shutdown in Oracle7, before migrating the database to Oracle8.