Any cbevents directories prefixed with cbevents* are used to create new cbevent partitions. Any cbevents directories prefixed with _cbevents* are used as read-only.
A read-only partition can load existing partitions, but new partitions are not created on it. You can use this approach when retiring old volumes. Old partitions will eventually be purged based on time. A second use for _cbevents prefix is for directories that are used only for "cold" partitions (old partitions that are only loaded on demand).