not only zpool as an upgrade also zfs

I knew about the upgrade of the zfs pool version via
zpool upgrade
But today I found, that also the filesystem itself has a version:
zfs upgrade

bash-3.2$ zfs upgrade -v
The following filesystem versions are supported:

--- --------------------------------------------------------
1 Initial ZFS filesystem version
2 Enhanced directory entries
3 Case insensitive and File system unique identifer (FUID)
4 userquota, groupquota properties

For more information on a particular version, including supported releases, see:

Where 'N' is the version number.
On my laptop it took some time, because the filesystem upgrade seems to
analyze the whole content of the filesystem.