You may deploy network architectures that use hardware or software to reduce the number of devices or users that directly access the software on a server. This is referred to as multiplexing or pooling. This does not reduce the number of CALs required to access or use the server software. A CAL is required for each device or user that is connected to the multiplexing or pooling software or hardware front end.
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, so SQL Server to SQL Server = no CAL required
An External Connector (EC) license is an alternative to CALs for each server that external users will access. External users are users who are not employees or onsite contractors. An EC license assigned to a server permits access by any number of external users, as long as that access is for the benefit of the licensee and not the external user. Each physical server that external users access requires only one EC license regardless of the number of instances running. The right to run instances of the server software is licensed separately; the EC, like the CAL, simply permits access. EC licenses, like CALs, are version and functionality specific. They must be the same version or later than the server software being accessed. The decision on whether to acquire CALs or an EC is primarily a financial one.
Bear in mind that EC will cover for example, WINDOWS SERVER for external non-employees
For SQL Server the choice is per PROCESSOR to cover internal AND external access, without having to count connections made directly or indirectly to the server, there is no SQL EC………..