A common pattern with beadm on FreeBSD is for the root filesystem dataset to be called "ROOT", and this mirrors what's in the main article. It appears this person has boot environments enabled, but didn't actually explain anything about them.

ZFS boot environments are awesome. You can get them on both FreeBSD and Illumos with beadm. On Linux there are a number of utilities, but I've only used ZSys. It comes working out of the box if you do a ZFS-on-root install with Ubuntu.

Here: https://is.gd/BECTL