Unity Products:Amplify Shader Editor/Light Attenuation
Light Attenuation Node
The Light Attenuation Node contains Unity's light and shadow information. With directional lights returns the white and black areas where the object is directly lit or in shadow, these change accordingly to your directional light settings. With point and spot lights it also contains the smooth falloff information of the light that changes accordingly to the range setting. In neither case contains light intensity or color information so usually it's becomes more useful when used in conjunction with Light Color node that contains both. It's only useful when doing some sort of custom lighting, thus the node is only visible in the available nodes menu if the Light Model is set to Custom Lighting and displays a warning if it ever happens to be outside of it.
NOTE 1: For simplicity sake when the light intensity is zero the attenuation is also zero.
NOTE 2: This node should be only used over connections made with the Custom Lighting input port from the surface Output node.