The Render > Fast Render sub-palette controls properties for the Fast renderer.
Ambient
The Ambient Amount slider, used only by the Fast Renderer, determines how much ambient (unshaded) light is rendered for all items in the canvas.
Diffuse
The Diffuse Amount slider, used only by the Fast Renderer, determines how much diffuse shading is applied to all items in the canvas.
Reference Guide > Render > Fast Render