Google Message Security for Google Apps Administration Guide - Manual (Page 25)

Organization Hierarchy & Design 25
Hierarchy Configurations
For the message security service to filter users’ email, it has to know about their
email address and their domain (the part of their address that comes after the @
sign). You register your users by associating them with organizations in your
hierarchy. When doing so, follow the best practice configurations described below.
Basic Hierarchy: One Domain / User Org
With one domain, and all its users have the same filtering, services, and
administrator, you can stick with the basic configuration set up when your account
was created—one account administrator org, and one user org with one
associated domain. As new users are created in Google Apps, they are added to
the initial user organization. Customize the user org’s settings and you’re done.
Basic Configuration: The Jumbo Users org has one domain, and all new users
are added to this organization. All users have the same filtering, services, and
administrator.