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"ArcGIS Server controls access to the GIS web services hosted on your server using a role-based access control model. In a role-based access control model, the permission to access a secured service is controlled by assigning roles to that service. To consume a secured service, a user must be a member of a role that has been assigned permissions to access it."
http://resources.arcgis.com/en/help/main/10.1/0154/0154000005m5000000.htm