Explanation/Reference:
D: Whenever data blocks change, the Change Tracking Writer (CTWR) background process tracks the changed blocks in a private area of memory.
When RMAN performs an incremental backup on the Exadata platform, cellsrv filters out unwanted blocks and sends back only those that have changed since the last level 0 or level 1 backup.
E: For the Exadata platform, you may choose to allow cellserv to do all the blocking filtering for incremental backups.
Note: RMAN block change tracking is used to improve the performance of incremental backup. The level 0 incremental backup scans the entire database but level 1 incremental backups use the block change tracking file to scan only the blocks that have changed since the last backup. This significantly reduces the amount of reads that are required on the database.
References: Expert Oracle Exadata (29 Aug 2015), page 319