Mental Ray Shaders (XSISDK)
A Mental Ray shader is defined with a SPDL, a Preset file (which is generated from the SPDL), and a .DLL (or .SO). XSI uses the SPDL file to get information about the type of the shader, and its parameters and is able to build a Property Page and a Render Tree node for the shader based on this information. Mental Ray will call the Shader DLL during the rendering process.
Be sure to read: mental ray Shaders (http://softimage.wiki.softimage.com/sdkdocs/cus_mrshad.htm)