This setting is available if the Threshold Type is set as Area. With this setting, you can adjust the opacity of the threshold’s inner area with a value between 0 and 100. 

• 0%: the threshold area fill will be completely transparent and not visible at all.
• 50%: the threshold area fill will be semi-transparent, applying a subtle shade over the chart elements and the background in your chosen Threshold Fill Color.
• 100%: the threshold area fill will be completely opaque, fully displaying your chosen Threshold Fill Color.