In this phase we describe the system requirements in terms of use cases in order to prepare the Agent Identification phase where these elements will be clustered and put under the responsibility of some agent. If a goal-oriented approach is used at the beginning of the design, requirements should be derived from it and then applied to this phase.
As a result, a functional description of the system is provided through a hierarchical series of use-case (UC) diagrams. Scenarios that are at the basis of the UC diagrams come from the Problem Statement document and they are explained using sequence diagrams in following phases.
The expected output of this phase is an UML use case diagram and a textual document containing the complete documentation of the use cases in terms of: name, participating actors, entry condition, flow of events, exit condition, exceptions and special requirements.
In the figure, an example of DRD diagram describes (some of) the activities of the PASSI design process (these could be thought as the description of the functionalities of a tool that supports the development process with PASSI).
It is worth to highlight that use cases identification, their refinement and relationships introduction is not considered within the scope of the PASSI methodology since we suppose this is part of the designer background skills and any of the existing approaches can be considered valid if it produces a good representation of the system requirements in this form.
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