Functional Layer is the Layer that looks at functionality of software systems. The audience at that level is quite happy to talk about screen forms, Web pages, as well as rather detailed data structures. However, no technical background is presumed, and no technical details are provided.
As the Functional Layer is tied to software systems, more than one model can exist in the Functional Layer. In addition to the Enterprise-wide Models, individual systems can have their own Data Models, Use Case Models, even Process Models.