Keylight creates the screen matte after the screen color has been picked. You can make fine adjustments to this matte using the Gain controls. Increasing the gain controls makes the screen matte more transparent by increasing the amount of screen color showing through the matte. This tends to tint the edges the opposite of the screen color (for blue screens, edges become yellow). Decreasing the gain makes the main matte more opaque by reducing the amount of screen color showing through the matte.
The matte can be adjusted independently in the shadows, midtones, and highlights, giving more control than the clipping levels.
The level of the midtones can be adjusted too. For example, if you are working on a dark shot you may want to set the midtone level to a dark gray to make the gain controls differentiate between tones that would otherwise all be considered shadows.