MULTIPLEXING: USE OF MIDDLEWARE, TRANSACTION SERVERS, AND MULTITIERED ARCHITECTURES
Sometimes organizations develop network scenarios that use various forms of hardware and/or software that reduce the number of devices or users that directly access or use the software on a particular server, often called multiplexing or pooling hardware or software. Use of such multiplexing or pooling hardware and/or software does not reduce the number of CALs required to access or use SQL Server software. A CAL is required for each distinct device or user that is connected to the multiplexing or pooling software or hardware front end.

This is true no matter how many tiers of hardware or software exist between the SQL Server and the client devices that ultimately use its data, services or functionality. An exception to this includes the manual transfer of data from employee to employee. For example, if an employee sends an Excel version of a report to another employee, the receiving employee does not require a CAL (as long as the report does not access a SQL Server in some way). An additional exception is communication exclusively between SQL servers.