Multiple Boot Environments
Beginning with version 9.1, PC-BSD supports multiple boot environments on systems that were formatted with ZFS during installation. In 9.1, this feature is configured from the command line. Version 9.2 will provide a graphical interface for managing boot environments (BEs).
Multiple boot environments is a feature originally created by Solaris. It allows you to create a BE, which is a bootable, point-in-time snapshot of the ZFS filesystem. By booting into a snapshot, you return to that point in time in the filesystem. For example, you could create a BE before upgrading, allowing you to boot either into the newly upgraded system or into the snapshot of the system before it was upgraded. Alternately, you could create a snapshot before installing and configuring some software that you wish to test. Once your test is finished, you could boot into what the system looked like before the test.