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).