In Autoscaling regime the color palette is automaticaly spreaded out to the whole range of height values on the image (the same will happen if you do the double-click on the palette in Fixed scale regime). You can specify a certain part of the image as a base for autoscaling – this will be the region of interest.

To specify the region, select it with one of the standart selection tools and then press the tool bar button:

You can also call this function from the menu View → Region of interest.

The selected area will be heilighted with red line. Use image settings to remove the line: