There are two important further considerations:

1. Confluence Administrator is another permissions role (and colloquialism) in Confluence. Like System Administrator, it can be applied to users or groups. The Confluence Administrator does not have full access to the complete set of administrative and configuration options. It is a more limited role than System Administrator. 

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   Please see the Global Permissions Overview page of the Atlassian Confluence documentation for a complete list of the differences between these roles.

2. There is an out-of-the-box Confluence group called confluence-administrators with the System Administrator role assigned to it. This group is, in fact, a set of superusers, and therefore should not be referred to as "Confluence Administrators," despite the group's name. For accuracy, any individual user in this group should be called a "System Administrator." (By contrast, you may create a different group, say, technical-librarians, that has the Confluence Administrator role and whose members could be called, accurately-speaking, "Confluence Administrators.")