I came across this and thought someone else might find it helpful:
Surface shaders -Surface shaders are attached to geometry. These shaders describe optical properties of geometric object. Simply said, Surface shaders tells the renderer what will the surface look like when it is hit by the light. Consider this shader, as the one that describes what will the object look like
Displacement shaders - These shaders change the topology of the surface. In order to change the topology, Displacement shader can do one of two things. It can do REAL displacement, which moves the actual geometry, or it can move the surface normals, which produce the infamous BUMP effect. These shaders are also attached to the geometry.
Volume shaders - Volume shader changes the color of the light ray as it travels through volume. Volume can be one of several things. For one, it can be geometry. If light ray travels through geometry and it has volume shader attached, this shader will modulate light ray’s color in a way you’ve described in shader. Secondly, volume can be light item. This means that you can have, for example, Spot Light’s cone represent the volume through which light ray’s will be modulated by a volume shader (volumetric lights). As for the third type of volume, if volume shader isn’t attached to anything, it will be considered as atmospheric volume shader. Consider fog, myst and similar effects.
Light shaders - This shader represents the light source. With light shader you describe the emission of light rays from the source (item that has the light shader attached) to the destination point, which is the surface being illuminated. These shaders generally give control over light color, shadows, intensity, falloff with distance, barn doors etc. Light shaders are shaders with which you control light sources. Important to note is that light source to which this shader can be attached to is either light item or geometry.
Imager Shaders - Imager shaders operate on image just prior to the final output. When renderer is finished with calculating pixel colors for the image output, this shader operates on these pixels. Look at this kind of shaders as MAX Render Effects. It’s a post-process shader, but with access to scene data (just like MAX Render Effects).
This doesn’t answer my question of the day, but it helps put some of the other docs I’m reading into context.