Retention rules are same for all the destinations.
A backup stored in a local or network folder cannot be deleted if it has dependent backups that are not subject to deletion. Such backup chains are deleted only when the lifetime of all their backups expire. This requires extra space for storing those backups whose deletion is postponed. Also, the backup age and number of backups may exceed the values you specify.
In single-file backup, the slices are marked for deletion instead of being immediately removed. Data is not removed from the backup file, marked blocks are overwritten with the next backup.
Retention rules are applied after the backup and can be triggered only by running the backup plan.