The Composition Definition role is responsible for defining compositions of elements for some purpose, typically to be built into artifacts. In practice, it is often combined with the Artifact Creation role, although Sepia regards it as a separate concern. Moreover, it is frequently not addressed by any dedicated service implementation, but through the configuration of activities in Activity Execution (i.e. as part of the activity script) or as part of the source code in a build file or dependency declaration.
None as of yet.
The Composition Definition role is usually invoked by the Artifact Creation role, which needs its resulting composition as input. This can either be done directly, or via the Activity Execution role.