For those operations that aren't allocated, they are then defined at the lower level as a collaboration between the entities at that level. This is done for the entities in each of the Viewpoints.

Elaborate the black-box flow-of-events text into sequences of white-box steps, which speak in terms of subsystem actions and interactions. Next, a System Operation (black-box step) is expanded into one or more white-box steps, each of which is performed by a named subsystem.

Note that the analysis now proceeds driven by System Operation; that is, treat the next realization step as the realization of each System Operation (rather than the more abstract notion of system use case black-box step). The white-box steps for each System Operation are captured (initially) and associated with the corresponding System Operation as its realization. The white-box steps are not stored with the System Use-Case, but can be traced from the System Use Case through the System Operation.