Under design control, the design phase should proceed until

In the design control phase of software development, it is critical to ensure that the design phase proceeds until system elements of the smallest structure are defined and specified. This ensures that all components of the system are thoroughly planned and documented, which helps in minimizing errors and misunderstandings later in the development process. Detailed design specifications provide a clear roadmap for developers and facilitate better quality control and testing.

References:

"Software Engineering: A Practitioner's Approach" by Roger S. Pressman

IEEE Standard 1016-2009 for Information Technology—Systems Design