The SharePoint Subscriber Access License (SAL) is to be used in case:
- There are only internal users (employees)
- The external users are countable and SALs can be assigned to specific people.
For each user accessing a SharePoint Server.
For each user using the Enterprise features of SharePoint Server (includes PerformancePoint Services, Excel Services, and Visio Services)
Enterprise SALs are additive. To access the Enterprise features, a user must have both the Standard SAL and Enterprise SAL.