Not sure, but it appears to be the 0.5 mV/m 50% Skywave for Class Is/As, and High Power Class IIIs, and 0.5 mV/m 10% Skywave for lower power stations. Keep in mind that very low power stations may not have Skywave returns above the threshold of either. Some drop below the threshold outside the major and large minor lobes of the DA, giving a vary narrow apparent beam width. Note that Class Is/As that fall below about 700 mV/m at 1 mile in any direction do not have 0.5 mV/m 50% Skywave returns.