A ZVOL is when you export your ZFS volume, or a chunk of it as a block device, which is needed for ISCSI.
What needs to be decided at this point is how many ISCSI volumes are going to be needed, as each client needs their own volume.
Below is an example showing how to make a 5TB ZVOL to be exported as a block device for ISCSI:
zfs create -V 5 T zpool/disk1
Now if you the ‘ lsblk ’ command again, you should see your ZVOL listed as device zd0, the next will be zd16, then zd32, etc.