System folders (also known as the Non IPM_Subtree) Users cannot access these folders directly. Client applications like Outlook use these folders to store information such as free and busy data, offline address lists, and organizational forms. Other system folders hold configuration information that is used by custom applications or by Exchange itself. The Public Folders tree contains extra system folders, such as the EFORMS REGISTRY folder, that do not exist in general-purpose public folder trees.
How do you view them?
In Exchange System Manager, you can view the system folders for a specific public folder tree by right-clicking the public folder tree node and clicking View System folders.
http://technet.microsoft.com/en-us/library/bb124189.aspx
