Very much like a building architect who comes along and designs the home of your dreams. We design the system of your dreams and just like a regular architect, we have no opinion on which builders or plumbers you use, or where you get your tools or materials from, however the end result should represent the original plan.
A common theme in our world is that the builders go off and construct their idea of what the house should look like. They move the doors and tear down walls and add rooms they think you will like (but serve no practical use).
If this happens in your systems design today then our method of detailed planning could work well for you.