 1) Region Ⅰ is the Dead Region (Cut-Off Region, Cut Out Region, or Turn Off Region), with an absolute value of voltage range of 0~U1. And in this zone, the SSR switch cannot be turned on, even if an input signal is added. 2) Region Ⅱ is the Response Region (Active Region, Cut-On Region, Cut In Region, or Turn On Region) with an absolute value of voltage range of U1~U2. In this zone, the SSR is immediately turned on, as soon as the input signal is added, and the output voltage increases as the supply voltage increases. 3) Region Ⅲ is the Suppression Region (Saturation Region) with an absolute value of voltage range greater than U2. In this region, the switching element (thyristor) is in the saturated state. More details via www.huimultd.com