When the sweep tone adjust button is tapped, a new screen is presented where you can customize the sound of the sweeping tone that is used to indicate the tuning error during normal tuning. There are four ways in which this tone can be customized. As these parameters are adjusted, a sweeping tone is sounded in the headphones so you can evaluate the adjustment according to your tastes.
Sweeping Speed. For a given tuning error, this setting controls how fast the tone sweeps up or down in pitch.
Tone Purity. The sweeping tone is deliberately impure to make it sound different from the piano tone you would like to hear while you are tuning. If you would like to make this sweeping tone more or less pure, you can do so with this adjustment.
Tone Pitch. The sweeping tone covers about one octave of pitch. This adjustment moves that octave up or down, according to your preference.
Volume. The loudness of the sweeping tone can be controlled here. This control is entirely separate from the normal volume control on your device. But the device's volume control will control both the sweeping tone and the VoiceOver volume. This sweeping tone volume control will only affect the volume of the sweeping tone and leave the VoiceOver volume alone. Therefore this volume adjustment is the way to control the relative balance between those two sounds.
In addition to four pairs of buttons controlling these four aspects of the sweeping tone, there is a button to reset all four parameters back to their initial defaults.