By adjusting the Span from each end, you can introduce Bias. The Bias represents a favoring of either lower intensity signals or higher intensity signals in the display. This feature is seldom used in practice, but you should experiment with it to determine if there are any situations where it can improve the results of your data.

While these descriptions highlight the various functions, the best way of learning is to use an image to experiment with the different settings. There is no “correct setting” as every operator’s eyes and preferences will be different, learn what works for you and use that as your start point when tuning your sonar picture.