This guidance explains how you use Base and Additive Client Access License (CAL) to license access to base and advanced Microsoft server software features and capabilities.

Client Access Licenses (CALs) are applicable to server products licensed under the Server/CAL and Per Core/CAL licensing models. Under these models you license your use of the server software via Server or (processor) Core licenses, and you separately license access to that server software via CALs. CALs themselves do not provide any software or services; they simply provide the right to access server software.

CALs are typically available on a per user or per device basis. User CALs permit the licensed user (to which the license has been assigned) to access the server software from any device, whereas Device CALs permit any user to access the server software from the licensed device (to which the CAL has been assigned).

Server software typically includes base functionality and advanced functionality. The required CAL for the server product depends on the functionality being accessed.

Base CALs are the minimum requirement for accessing any of the applicable server product’s functionality. For example, anyone in your organization accessing any Exchange Server functionality requires an Exchange Server Standard CAL, which is the Base CAL for Exchange Server and Windows Server CAL, which is the Base CAL for Windows Server.

Access to advanced functionality associated with an Additive CAL requires the appropriate Additive CAL in addition to the Base CAL. 

For example, accessing in-place archiving or holds or any other additional functionality associated with Exchange Server Enterprise CAL (the Additive CAL for Exchange Server), requires both Exchange Server Standard CAL and Exchange Server Enterprise CAL. Likewise, remote access to Window Server requires Windows Server Remote Desktop Services CAL (an Additive CAL for Windows Server) in addition to Windows Server CAL. You can find the information in the Product Terms Additional Functionality Associated with Windows Server 2025 Remote Desktop Services CAL.

Base and Additive CALs are tied to the specific functionalities they enable access to and not the different editions of the server software. For example, Exchange Server Standard CAL licenses access to base functionality whether provided via Exchange Server Standard or Exchange Server Enterprise, and Exchange Server Enterprise CAL licenses access to additional functionality whether provided via Exchange Server Standard or Exchange Server Enterprise. In this sense, Standard CALs and Enterprise CALs are associated with base functionality and additive functionality respectively as opposed to being associated with the Standard and Enterprise editions of the server software.

You can determine the Base and Additive CAL requirements by consulting the Access Licenses section of the Product Terms for the specific server product. This section provides detailed information regarding the applicable CALs—both Base CALs, which grant access to standard functionality, and Additive CALs, which are required for advanced features. It also outlines the specific functionalities tied to additive CALs.

For instance, the Access Licenses section for Exchange Server specifies the Base CALs necessary for standard functionality, such as Exchange Server Standard CALs or equivalent User Subscription Licenses. It also highlights advanced features, such as in-place archiving, holds, and data loss prevention, which require Additive CALs like Exchange Server Enterprise CALs or their equivalents.

While Exchange Server serves as one example, the Product Terms feature similar sections for other server products, providing tailored guidance on CAL requirements and associated functionalities for each. By reviewing these sections, you can identify when Base and Additive CALs are needed and the features they unlock for various server products.

The tables in the Access Licenses section of the Product Terms for the specific server product also reflect that you have multiple options for fulfilling the CAL requirements, including: 

1. CALs for individual server products. You can purchase individual, standalone CALs for a server product. For example, you can see from the tables above that Exchange Server Standard CAL meets the Base CAL requirements for Exchange Server and Exchange Server Enterprise CAL meets the Additive CAL requirements. 
    
2. CAL Suites. Instead of buying individual CALs, you can opt for a CAL Suite that bundles several CALs (and MLs) together. The Core CAL Suite is primarily composed of Base CALs and the Enterprise CAL Suite includes all those same Base CALs as well as Additive CALs. For example, the Core CAL includes Exchange Server Standard CAL along with Base CALs other server products and the Enterprise CAL Suite includes both Exchange Server Standard CAL and Exchange Server Enterprise CAL along with base and Additive CALs for other server products. Whether purchased individually or through a CAL Suite, the access rights are the same. Refer to the CAL and ML Equivalency Licenses section of the Product Terms to see which CALs and MLs are included in the Core CAL Suite and Enterprise CAL Suite, and to the CAL Suites licensing guidance for more information. 
    
3. User Subscription Licenses for individual online services. User Subscription Licenses (SLs) for certain online services also meet the base and Additive CAL requirements. For instance, the tables above reflect that Exchange Online (Plan 1/1G/2/2A/2G) User SL meets the Exchange Server Base CAL requirement, and an Exchange Online (Plan 2/2A/2G) User SL meets the Exchange Server Additive CAL requirement. 
    
4. User Subscription Licenses for online services suites. Like CAL Suites, you can also purchase User Subscription Licenses for a suite of online services which provide access to server products in addition to the included online services. For example, Microsoft 365 E3 includes Exchange Online Plan 2, so it meets the base and Additive CAL requirements for Exchange Server. Refer to the CAL and ML Equivalency Licenses section of the Product Terms to see which CAL and ML equivalents are included in the various online services subscription suites.

The Use Rights section in the Product Terms for server products provides crucial details regarding the requirements for licensing server access for External Users (as defined in the Glossary of the Product Terms). This section outlines the conditions under which external access is licensed and highlights any additional requirements for accessing advanced functionalities.

For example, in the case of Exchange Server, the Use Rights section specifies that external user access to the server's base-level functionality is included with the Exchange Server license itself, meaning Base CALs are not separately required for external users. However, to access advanced features, as defined in the "Access Rights" section (such as in-place archiving, holds, or data loss prevention), external users must also be licensed with both a Base CAL and an Additive CAL.

This structure ensures that organizations can effectively manage external user access while tailoring licensing to the functionality being utilized. Similar requirements and licensing structures apply to other Microsoft server products, and reviewing the relevant Product Terms for each product will provide clarity on specific Use Rights and external access needs.

For some server products, you have the option to license all access by external users through one license per server called an External Connector license. For example, the Use Rights section of the Product Terms for Windows Server Standard, Datacenter, and Essentials lists “CALs or External Connector” as options for meeting External User Access Requirements. Under Server External User Access in the Access License section, you will find the specific External Connector licenses associated with the Base and Additive CALs for Windows Server.

When licensing access to Microsoft server products, you need to license access to both the server operating system (Windows Server) and the server applications (such as Exchange Server, SharePoint Server, etc.) being accessed.

See FAQ page.