What external capacity tiers and tiering policies are

External capacity tiers provide storage for infrequently accessed data. You can attach an all flash (all SSD) aggregate to an external capacity tier to store infrequently used data. You can use tiering policies to decide whether data should be moved to an external capacity tier.

You can set one of the following tiering policies on a volume:

Snapshot-only
This is the default tiering policy. This tiering policy enables you to tier the Snapshot data of only those volumes that are currently not being referenced by the active file system.
None
Prevents the data on the volume from being moved to an external capacity tier.
Backup
Enables the newly transferred data of a data protection (DP) volume to start in the external capacity tier.