The hyperbola is used to define the performance margin. Systems whose performance degradation curve intersects high on the hyperbola curve represent "non-robustly performing systems" in that very small deviations of the uncertain elements from their nominal values can result in very large system gains. Conversely, an intersection low on the hyperbola represent "robustly performing systems."
The point where the system performance degradation curve crosses the green line is used as a scalar measure of the robustness of a system to uncertainty. The horizontal coordinate of the crossing point is the robust performance margin, PerfMarg.