The waterfall lifecycle model is most appropriate to use when the requirements are well known and unlikely to change. This model follows a linear and sequential approach where each phase must be completed before the next one begins. Because it assumes stable and well-defined requirements, it allows for thorough planning and design upfront, which can lead to a more structured and predictable development process. Waterfall is less flexible in accommodatingchanges compared to iterative or agile models, making it suitable for projects with clear, stable requirements. This approach is detailed in traditional software engineering textbooks and standards such as the IEEE Standard for Developing Software Life Cycle Processes (IEEE 1074).