Value To

This setting will be available if you have selected Area as your Threshold Type
Area threshold type requires two values to determine the start and end of the area. While the starting point is determined by Value From (or calculated dynamically, based on your Value Type), Value To property governs where the area ends. 

Regardless of your chosen value type, you can enter a constant numerical value in the Value To field which will be used as the end point of the area threshold. 
If you provide only one value for your area threshold (either in Value From or Value To settings), the threshold will be displayed as a line using the value that you have provided.