From Oracle FAQ
CTWR (Change Tracking Writer) is an Oracle background process created when you start an instance with BLOCK CHANGE TRACKING enabled. This feature improves incremental RMAN backup performance by recording changed blocks in each datafile in a tracking file. RMAN uses the tracking file to identify changed blocks for incremental backup, thus avoiding the need to scan every block in the datafile.
SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE os_file_name;
SELECT filename, status, bytes FROM v$block_change_tracking;
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