In the second stage, the prototype stage, the requirements and the requirement document are frozen and placed under change-order control. Changes in requirements are still allowed but should be very rare. Any new requirements after this point are very costly. Only if the requirement change is absolutely necessary to the success of the product, despite the potential delays in the product delivery or cost over-runs, is the requirement change allowed. The main idea is to force control on any new requirements. This forces the cycle to be completed and enables product delivery. The architecture is still allowed to vary a little as technology pressures deliver new options.
Once the critical tasks are done well, the implementations associated with the critical tasks are expanded to cover more and more of the application.
One of the goals of this stage is for the team to convince non-team members that the solution can be accomplished.
At the end of this stage, the process is ready for transition into the alpha and beta release stages.