General/SystemUIServer runs the System Menu Bar.

Or at least the part on the right that handles General/NSMenuExtras and General/NSStatusItems (including the Spotlight menu). The part on the left is the application menu bar.

Other (not so important to Cocoa) duties that it takes care of include launching applications when General/CDs and General/DVDs are inserted, and putting up the dialog that says “You’ve inserted a blank CD, what would you like to do with it?”

If it crashes or is killed, it will automatically relaunch.