With most Microsoft servers in a virtualized environment, there are two types of licenses that you can, and in some cases must, acquire – Client Access Licenses (CAL) and External Connectors (EC). The requirements for both are the same – one is required for each device that accesses an instance of server software. This can be on either a physical or virtual server.
For example, if you have an employee who needs access to the physical server on some occasions and also requires access from an external location (they travel frequently, work from home, etc) the server must have both a CAL and an EC license. You may also reassign EC licenses to different users and servers, within a 90-day period. There are exceptions to this rule, such as in a server farm.
EC licenses have become a necessity as more workforces move to the cloud and have greater mobility that they did 10 years ago.