In a full resynchronization, RMAN updates all changed records, including those for the database schema. RMAN performs a full resynchronization after structural changes to database (adding or dropping database files, creating new incarnation, and so on) or after changes to the RMAN persistent configuration.
When a database is registered with a recovery catalog for the first time a full catalog synchronization is performed
https://docs.oracle.com/en/database/oracle/oracle-database/19/bradv/managing-recovery- catalog.html#GUID-6B077FE7-8E10-4603-8FDB-CAE3357E85C9