The button 
allows you to defined the ground level in relation to the global coordinate system. This way Diamonds knows:
- how much soil you dig down when calculating the soil layers.
- how many meters above the ground level an element is located when generating wind.
By clicking on the icon 
you can choose to visualize the ground level in your model or not.
Remarks:
- The soil calculations and the generation of wind loads don’t have meaning if you haven’t set the ground level!
