The project manager needs to lead the project team in planning to determine scope. This is typically done by constructing a WBS. But the project manager needs to focus on the big picture, and even though the WBS has sufficient detail to determine the estimates for resources, budgets, and effort, there may be many different interpretations of what is in and out of scope.
The business systems analyst helps with the detailed definition of work packages by developing by producing context diagrams and use cases. In other words, they focus on who will use the system and what will their experience be. Context diagrams are high level views of how the components of a system will interact together. For those using OOD, these may be high level Entity Relationship Diagrams or ERDs.
Use cases show what services the system will provide and to whom. It includes the humans or "actors" that will participate in or benefit from the system.
Together these tools will provide a means of communicating the proposed solution to the project sponsor and/or other key stakeholders that will approve the scope. To aid in this analysis, it is likely a project communications plan has already been started by the PM. This document should identify all the key stakeholders and that list will be important in determing who would benefit from a review of these diagrams.