Shader Channels

From ZBrush Info

The Shader channels of a material are where different material modifiers are grouped together. The types of modifier and their settings determine how the material will appear when it is rendered. When a material has more than one Shader channel, the effects are combined. There can be up to four Shader channels, as indicated by the buttons S1 to S4.

For example, the FastShader material has a single Shader channel (S1) that has only two modifiers, Ambient and Diffuse. Changing the Ambient setting will change how light or dark the object is when rendered, while changing the Diffuse setting will change the amount of contrast between lit and unlit portions of the object. Remember these are not changes in lighting; nothing else in the scene will be affected. You are simply changing the way that the object's surface responds to the lighting of your scene.

The BasicMaterial forms the basis of many of the standard materials. Its Shader channel will always be the last one, so for a material with two Shaders it will be S2; for three Shaders, S3.

A number of Shader settings use two modifiers: a numeric parameter that determines the overall strength of the setting, plus a curve parameter that lets you use a graph to specify and to vary the strength according to the direction of each affected pixol's surface normal (the Noise curve is an exception to this).

To Learn About Using Curves Click Here.