Phenomena are mental ray for Maya components that look and behave like shaders but, unlike custom shaders, they require no programming knowledge of C or C++ code to create.
By exporting an assembled shading network (or portions of it) and designated attributes, you create a Phenomenon, a single node that you can then use just like a regular shader: you can build shading networks with them; save, load, and export them; write your own Attribute Editor templates; and provide icon, swatches, and help files. See the devkit for more information and examples.
Previously generated phenomena
In order for the Node Factory to find and generate in Maya previously exported Phenomena, you need to export the file (for example, my_phenomenon.mi) to the directory specified in the environment variable: MI_CUSTOM_SHADER_PATH. The default value of this environment variable is: C:\Program Files\Autodesk\Maya2010\mentalray\include.