In previous database releases 19c,18c,12cR2 when you attempted to upgrade the recovery catalog through the command:  

UPGRADE CATALOG

You might face a problem that there are LIVE connections coming against the recovery catalog which will prevent you from upgrading the catalog…recovery catalogs are usuually upgraded after database upgrades and patches so it will be a continous process over time.

In Oracle 26ai a new command has been introduced —-> “TERMINATE” to terminate live sessions connecting to your recovery catalog and ease your catalog upgrade process while enabling “maintenance mode” in the recovery catalog.

RMAN> SET CATALOG MAINTENANCE ON;

CONNECT CATALOG rco@catdb;

TERMINATE BLOCKING CONNECTED USERS;