In real world projects, I think these line of phase processes act more as a framework rather than a strict sequence of steps, but alternatively they should still be linear in some fashion. As several others have mentioned, teams often need to revisit earlier phases when there is an unexpected issue during, let's just say, the execution phase, such as technical problems or resource limitations. In engineering and medical device development, testing or prototype results could reveal problems that require going back to planning and adjusting requirements. Because of this, processes can become more iterative rather than linear, but the structure is still vital to teams. While I don't have experience in medical device development in the real world, I do have experience working with a multitude of teams with a similar format and if these processes aren't followed or communicated properly, the goal or mission did become more difficult.