For an automatic snapshot, the number of days for which ElastiCache will retain the snapshot before deleting it.
For manual snapshots, this field reflects the SnapshotRetentionLimit for the source cache cluster when the snapshot was created. This field is otherwise ignored: Manual snapshots do not expire, and can only be deleted using the DeleteSnapshot action.
Important If the value of SnapshotRetentionLimit is set to zero (0), backups
are turned off.