Ideally, the WBS should be created from a/the Statement of Work (SOW). Though this might be more applicable to aerospace/DoD projects, commercial projects may also employ a SOW or similar methodology to provide projects with formal user requirements. The sections in the SOW would link directly to the applicable sections in the schedule.
Further, the WBS ought to be directly linked to the cost management system as well (accounting charge numbers, project numbers, work packages & etc.).
Thus, an ideal WBS represents the project structure, from the customer’s point of view, and the program management/scheduling/cost management point of view(s).
I insist on a deliverable-based WBS. If the task doesn’t support an articulated and approved deliverable of the project, it doesn’t belong in the WBS; and if it’s not in the WBS, it’s not in the scope of work. Unless I misunderstood your question.
I’m with Sarah. We start with the deliverables of the project. Deliverable, however, is a loose term. A deliverable might be a report, a white paper/study, a product, a meeting, or a service. And as Josh said, you will find all of the key deliverables in your SOW.