In the third stage, the architecture is frozen and placed under change-order control. This means that no more architectural changes are allowed unless they are absolutely necessary. Emphasis is now placed on the implementation and quality assurance.
The first version in field release is usually called an alpha release, while a second release is called the beta. The product may be immature in the alpha release. Only critical tasks have been implemented with high quality. Usually, only a limited number of customers are willing to accept an alpha version of the product and assume the associated risk.
During the beta release, enough of the system should be working to convince the customer that soon the beta application will be a real product. The beta release is more mature and is given to a much larger customer base.
When enough of the system is built, the system is ready for a transition into the next stage: releasing a high quality product.