Microsoft server Base and Additive Client Access Licenses (CALs) overview_October 2020
Base and Additive CALs – Details
Software dependency license requirements
There are two practical components when licensing access to Microsoft server products: access to the Windows
Server operating system itself, and access to the Microsoft application that works within the Windows Server environment. Customers must license access to each layer of the stack. Common examples of this include use of
Exchange Server and Skype for Business Server. Customers must acquire CALs for Windows Server, which is required
for use with these products, as well as the appropriate CALs for Exchange and/or Skype for Business Server.
Server software generally includes base functionality and advanced functionality. Base server functionality can be
accessed with a base CAL and additive CALs license access to advanced server functionality and must be acquired in
addition to the corresponding base CAL. Both types of CALs are generally available on a per-user and per-devicebasis.
Under the Server/CAL and Per Core/CAL licensing models, access to base functionality of a server product requires a
Base CAL for each accessing user or device..
Some Microsoft server software products, such as the Windows Server operating system, license additional
functionality with licenses known as “Additive CALs.” Additive CALs license advanced functionality in the server
software and must be acquired in addition to the base CAL. Additive CALs are sometimes referred to as “premium”
or “premium services” CALs.
Examples of Additive CALs include the Windows Server Remote Desktop Services CAL and the Windows Server
Active Directory Rights Management Services (RMS) CAL. Both of these Additive CALs license advanced
functionalities included in Windows Server that each device or user must be licensed for in order to access the
advanced functionality. The following figure describes the four licensing options with Windows Server CALs
Microsoft Exchange Server offers an Additive CAL called the Exchange Server Enterprise CAL, which is licensed in addition to the Exchange Server Standard CAL. Each user or device requiring advanced functionality from the Exchange Server Enterprise CAL also needs to purchase a matching Exchange Server base Standard CAL as a prerequisite.
The following figure describes the two licensing options with Exchange Server CALs.
In a similar fashion to these examples, Microsoft SharePoint Server offers the SharePoint Standard CAL and the SharePoint Enterprise CAL. Skype for Business Server offers the Skype for Business Server Standard CAL, the Skype for Business Server Enterprise CAL, and the Skype for Business Server Plus CAL.
External user access
Microsoft server software requires access licensing for external users. Some server software licenses external user
access with the server license itself and no other access licensing is required. For server software that requires Base
and Additive CAL access licensing, an External Connector license may be used to license all external users. An
External Connector license is assigned to the server and licenses all external users instead of requiring a CAL for
each external user. Server software that includes base and advanced functionality will still require the base External
Connector license and any additive External Connector licenses for any advanced functionality enabled
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