This is a very useful area, and is the only way of doing certain tasks. When you do system upgrades or move storage between systems, this is where you reconfigure the disk ownership or change the boot aggregates.
When the system boots, it says “CTRL+C for Special Boot Options”, do this, and you will later be given 6 choices.
(2) Boot without /etc/rc.
(3) Change password.
(4) Initialize owned disks (5 disks are owned by this filer).
(4a) Same as option 4, but create a flexible root volume.
(5) Maintenance mode boot.
Option (5) gives us the Maintenance Mode command line. There are several commands available here, and I will post on each one separately to allow people to comment on each, and for me to update with ways of doing things under each command.
I may not have chance to go through all the commands, but I will pull them apart as I have time. These pages should be useful as a reference to what might be available to you.