Any backup plan where the retention rule is set contains a cleanup task in addition to a backup task.
The cleanup task checks the archive created by the backup plan for backups that have outlived their
lifetime. If such backups are found, the task makes the cloud storage delete them. Since the deletion
is performed on the cloud storage side, it does not take your machine's CPU resource.
The cleanup task runs after every cloud backup, even if the backup has failed. The last successful
backup is always kept though. For more information about the retention rule please refer to "How
long will my backups be kept in the cloud storage?" (p. 4)
Normally, there is no need to start and stop the cleanup task manually. But it is possible to do so in
the Backup plans and tasks view.