These commands aren't specific to BSD Jails and there are no nested pools here, just a single pool.
Under ZFS, you can create as many datasets as you like in a pool. These datasets can be either volumes or file systems. Here two extra file systems are created. They are laid out a hierarchical manner so here are the three file systems present in the pool:
zroot zroot/jails zroot/jails/fulljail1 and their mount points are:
/ /usr/local/jails /usr/local/jails/fulljail1 Under ZFS, creating a file system is a much lighter operation than with traditional file systems as there is no need to have a dedicated volume for it. All file systems share the same disk space. Creating a file system is nearly as lightweight as creating a new directory but has many advantages comparing to mkdir. For example you can create snapshots, clone, send, receive, set properties like compression or case sensitivity, mount elsewhere a zfs file system.