System Access allows a user access to all items on the client computer, including the Finder and the System Folder. When a user with System Access logs in to a client computer, System Access appears as a choice in the workgroups log-in window.
Global Administrators always have System Access. Other users only have System Access if a Global Administrator sets this option for them.
Having System Access is equivalent to pressing command-shift-escape from the At Ease/Macintosh Manager login window. You will be required to enter an administrator's password if there is one.