No. CAL requirements differ among server products. The general rule is that you must acquire and assign a CAL to each device or user that accesses your server software. Beyond that, however, there may be product-specific exceptions to that rule that affect a given product’s CAL requirement.

With Exchange Server 2013, for example, CALs are not required unless the server access is directly or indirectly authenticated by using Active Directory. For Windows Server, a CAL is required for all users or devices that are accessing the server. The CAL must be of the same edition or later; however, for Windows Server 2012 R2, you can use a Windows Server 2012 CAL to access the server.

For complete information about the different Microsoft CAL requirements, see the Microsoft Product Terms.

Generally, additive CALs can access any edition of server software. For example, Exchange Server 2010 and Exchange Server 2013 are available in both a Standard Edition and Enterprise Edition. The Exchange Standard CAL and Exchange Enterprise CAL may be used with either edition of the server software.

For more information, download the Volume Licensing brief for Base and Additive Client Access Licenses.

The CAL version must correspond to the server software version that it accesses. Older versions of CALs cannot be used with the newer version of the server software, but newer version RDS CALs can be used with an older version of the server software as defined in the RDS and TS CAL Interoperability Matrix.

The only exception to this rule is the R2 server releases; the older CALs sometimes work with the newer R2 release of server software. For example, there are no new Windows Server 2012 R2 RDS CALs required, so the current requirement is that you need at least a Windows Server 2012 RDS CAL to access RDS on Windows Server 2012 R2 servers.