A Domain is an collection of Workgroups created by the System Administrator. Resource types and Templates that are assigned to a Domain will be visible only to workgroups belonging to that Domain. If you are a Domain Administrator, you will be able to edit some of the Domain properties from the User Web Interface. When you create a template, the Domain selected will determine who in the organization has access to the template.
For more information see the documentation for Domains in the Administrator Manual
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