Pressing this button (located next to the "Record Chord" button brings up a toolbox that allows you to view a scale on the fretboard. Simple choose a note and a scale, and the fretboard will display all the notes within the scale on the fretboard.
When a scale is present, you can highlight any note within the scale.
Also, you can choose between Sharps and Flats by using the radio button.
If you no longer want to view the scale press the button "Turn Off Scale Control". The fretboard will clear.