In the development of software with evolving requirements, activities of requirements-engineering and management are present through the whole software development process and affect most of the actors involved. This paper presents a conceptual model which aims to an efficient requirements management by offering a central data structure for requirements that integrates requirements, design and implementation workflow. This conceptual model was developed for change-intensive embedded systems, but can be easily adapted to other domains. This paper presents the model itself, techniques to adapt this model to specific needs, and demonstrates its usage. Keywords Software Requirements, Software Evolution, Requirements Tracing, Software Product Lines