Most process standards agree that there are two ways that processes can be applied. These two ways are called recursion and iteration.

Recursion occurs when a process is applied to successive levels of system elements within a system structure. The outcomes of one application are used as inputs to the next level in the system structure. For example, the verification process is designed to apply to the entire assembled product, the major product components, and even components of components. How far into the product you apply the verification process depends entirely on the size and complexity of the end product.

Iteration occurs when processes are repeated at the same system level. New information is created by the implementation of one process that feeds back into a related process. This new information typically raises questions that must be resolved before completing the processes. For example, iteration will most likely occur between requirements development and technical solution. Reapplication of the processes can resolve the questions that are raised. Iteration can ensure quality prior to applying the next process.

Engineering processes (e.g., requirements development or verification) are implemented repeatedly on a product to ensure that these engineering processes have been adequately addressed before delivery to the customer. Further, engineering processes are applied to components of the product. For example, some questions that are raised by processes associated with the Verification and Validation process areas may be resolved by processes associated with the Requirements Development or Product Integration process area. Recursion and iteration of these processes enable the project to ensure quality in all components of the product before it is delivered to the customer.