Problems and requirements [software development]

Requirements, specifications, and programs are distinguished by the phenomena they concern. Requirements are about phenomena of the application domain and describe properties of the domain that the machine is required to bring about and maintain. The application domain is informal, and serious difficulties are encountered both in describing it and in reasoning about it. Requirements are complex, so they must be decomposed. Decomposition is based on the recognition of simple subproblems, characterised by problem frames.