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The three special groups in Confluence are:
All users who don't log in when they access Confluence are know as 'anonymous' users. By default, anonymous users don't have access to view or change any content in your Confluence instance, but Confluence admins can assign permissions to this group if it's required.
Overlapping group and user permissions
When a user is assigned more than one permission, the more powerful permission will prevail.
Further explanation: