The user interface layer is a consumer of services, just like your BPM component, but it can also be a service in itself.
Services are often regarded as automated components without a graphical user interface such as web services. However, services can have any interface, also a user interface. An example is a service
PrintService that provides a portlet interface for creating print tasks. Another example is any app from any app store.
In silos, users have to switch between using different user interfaces during the execution of a single task. For example, an employee in your HR department might need to switch between her email client, HRM application, financial application, and network file system containing documents to finalize ...