The worst-case gain measure is the maximum achievable system gain over all uncertain elements whose normalized size is bounded by 1.
On the graph, this is the vertical coordinate of the performance degradation curve as it crosses the vertical line drawn at the uncertainty bound = 1.
Each measure captures a single scalar attribute of the system performance degradation curve. Mathematically, they are independent quantities, answering subtlely different questions. Consequently, for two uncertain systems, sysA and sysB, it is possible that the StabMarg of sysA is larger than the StabMarg of sysB, though the PerfMarg of sysA is smaller than the PerfMarg of sysB. Nevertheless, they are useful metrics for concise description of the robustness of a system (uss or ufrd) due to various uncertain elements.