ZFS "list" command - USED / REFER capacity information
For ZFS and the `zfs list` command, "REFER' describes the amount of "live" data that is logically visible to clients accessing the filesystem.
For example, this is what you would see as "Used" from a client side command such as `df -h`.
"USED" describes the amount of data physically consumed(after compression) on the Storage Pool and disks by the data.
So, if you have a Storage Volume with nightly snapshots that contains a database dump that is overwritten or heavily modified every day, the "REFER" value will remain relatively static, but the amount of data "USED" for the snapshots and parent Storage Volume will grow over time.
As you delete older snapshots, this will release blocks which will reduce the amount "USED".