tty@tty12 is the standard tty available on Obarun. It is enabled by default on boot tree and its definition is contained within the boot@-66serv package.
One of the reasons this entry is created is to alert users of its peculiarity which makes it unlike most other unix/linux systems. If no other tty is enabled, on any other active tree, the only way to physically login to the system is through tty12. You can not use root as the login name to enter the system, only a user can. The reason for this is safety. Let's say you have installed this system in a remote server that you only access remotely, through ssh for example. To guess both the user name and password is much harder, therefore more secure. Other login terminals are not necessary for a remote server system.
In case a system administrator decides to make an installation of a system without a user and use the root, if something goes wrong with the booting process, like a disk volume fails to mount, the sys-admin will get no other tty and tty12 will not allow anyone in. So keep this security feature in mind before you lock yourself off the system and have no other medium to boot the machine other that this installation.