In fact, relying solely on the above SNDSM still cannot sort solutions in the same grade set, we call these solutions undetermined solutions (USs), and we further rank them according to the single congestion degree (SCD). Fig 6 shows the example of SCD. Define the ultra-optimized solution for each solution in US set, whose rank of each objective is immediately lower than the solution, for instance q' is the ultra-optimized solution of q. The boundary consisting of those ultra-optimized solutions are called ultra-optimized boundary, namely A'. SCD is proposed to measure the closeness between a solution and its ultra-optimized solution. Intuitively, SCD is the perimeter of the rectangle in Fig 6, which is composed of the solution point (i.e. q) and its ultra-optimized solution point (i.e. q') on the ultra-optimized boundary. For a solution in US set, the less SCD means more optimal, and result in lower ranking.