The waterfall model is based on milestones. For a milestone to be achieved, all the tasks related to that phase must be completed. The next phase of development cannot be started until the milestone from the previous phase is completed. The clearly identifiable milestones of this model make it is easy to monitor the progress of the project and the schedule, and to assign responsibilities and accountability to the appropriate resources. The waterfall model is more applicable to projects that have clearly defined requirements and are not liable to modifications in the future.
The spiral model was created with the intention of being able to refine the product requirements and project estimates on a regular basis. Each time the project goes through an iteration of the spiral model, the development team can assess the project and plan for the next iteration. However, because the spiral model has no clear checkpoints, monitoring the progress of the project is difficult. The spiral model is suited best for rapid development of small projects.
The MSF Process Model incorporates the milestone approach of the waterfall model with the iterative approach of the spiral model. By using the process model, the development team takes an iterative approach to building a solution, while tracking the progress of each iteration by using milestones.