There is a conversation on LinkedIn around the structure of the WBS. Neil Albert, of MIL-STD-881-C, provided ini nice answer to the question about the product or functional structure of the WBS. In the business of building things that swim, fly, drive - the WBS should be product based. But there are other domains where the outcome of the program is not a thing but a service, a result of the service, a repaired product, or even the absence of things in the case of environmental cleanup.
In all cases the WBS answers the question (along with several other artifacts) "What Does Done Look Like?"
At the bottom of the WBS is usually where the actual work takes place to create the evidence of "done."
This is a "deliverables based" description. Once the evidence of the deliverable is defined in the WBS Dictionary, you can then measure your progress toward that deliverable with EVM. So instead of using products, services, or functions as the starting point start with "done," then those three will emerge as you search for the "tangible evidentiary materials," needed to recognize "done."
