This SHOP provides an area for creating RIS shader nodes. This SHOP is not a shader. It is a container for shaders to be used in the RenderMan RIS rendering mode. Since it is not a shader, it cannot be assigned to a geometry as a shader, but its children can.
RIS shaders are different from traditional RSL shaders, because they define a BxDF rather than a color. Some intermediate nodes may actually compute color, but the assignable RIS shader node will provide a BxDF.
RIS also allows shader layering by wiring outputs (export parameters) of one shader to inputs (parameters) of another. Such shader composition is similar to co-shaders. The co-shaders are not called explicitly in the main shader, but are invoked by the renderer and the results are passed in as main shader parameters.
Please see RIS shaders for more information.