This illustration shows a timeline moving forward with the following three archived redo log files on it:

There are other archived redo log files in between the ones shown on the timeline. At the required checkpoint SCN, the capture process computes the age of checkpoints, with the following results:

The checkpoint retention time is set to 20 days. Therefore, the checkpoint that corresponds with the archived redo log file with sequence number 200 is purged, but the checkpoint that corresponds with the archived redo log file with sequence number 220 is retained.