Most Microsoft server products are paid for when you have bought the product itself and no user authentication is involved. Some server products however must also be paid for the individual (device or user) usage. Microsoft has good discussion of this topic at: http://www.microsoft.com/licensing/resources/downloads/default.mspx Look for "Server Customer Guide"
Any user added to the Enterprise Process Center who is recognized by the network domain controller. These users may be imported from Active Directory or added manually. Authenticated users automatically belong to both the Everyone and Authenticated Users groups.
A user that has presented valid and accepted login credentials to a resource controller.