Site user

From AgileApps Support Wiki

A Site is a collection of JSP pages that can be accessed by external users. You can still require Site users to register, but unlike case agents or designers, they don't count as "internal" users. (There is a charge for internal users. There is no charge for site users.)

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Note:
The Service Portal is an example of a Site that was created for Case Management applications. So portal users are another example of "site users".