We build an MDA process so that we know what models to build, which mapping functions are required, and what marking models to apply. This MDA process for your project is a living entity; as your understanding of the system grows, the choice of models and links between them may change and grow. It provides a framework for the work that needs to be done, and it can be used to capture revised understanding and act as overall top-down plan for the project. You have to have a plan so you can change it.
Chapter 12 takes up the topic of using the MDA process you have built.