How does “multiplexing” affect license usage of the Microsoft CAL (Client Access License)?
In principal, you are required to have a CAL for every direct or indirect access to a server application (where access to the server is licensed per CAL).
That means that every user/device accessing files, data, or content on the server requires a CAL (direct access).
A CAL is also required when the retrieved data is forwarded via an automatic step (for example, from an SQL Server to an application that the user is working on and thus the user uses the SQL Server in the back-end), to a user/device ( indirect access). In cases where data is provided by means of a manual step (for example, when a user processes and forwards information from the server application), the corresponding receiving user/device does not need a CAL.
Source: Volume Licensing Brief_Microsoft_Multiplexing 2016-09-19