Options area in site admin is where you can configure the important options for your site – like whether people can freely sign up for blogs, if you want your site to be private and what emails you want to send out to new users.
You access options by clicking on Site Admin > Options.
Remember to click Update Options at the bottom of the page when you make any changes.
Your type of WPMU installation determines what site admin options are included.
‘Out-of-the-box’ wp.mu, where the installation of WordPress MU is done for you, includes every WPMU DEV Premium plugin offered on the site alongside a number of extra ones combating spam and adding features such as site maintenance… these are the premium site options
See next: Standard site options