Mia stands for Mental Image Architecture, it is the most used material to shade hard surfaces like wood, glass, metal, and stone. While there are 3 different Mia shaders, (mia_material, mia_material_x, and mia_material_x_passes), it is most convenient to always use the latter one, as the previous versions are identical except for the fact they miss some bump and render passes features.
A major advantage of Mia compared to standard Maya shaders like Blinn, is that it is energy conserving, meaning that the sum of its values (diffuse, reflection, refraction, translucency, etc.) never add up higher than 1. Suppose you have a Blinn shader without reflections and diffuse=1; if now you increase its reflectivity to 0.4 you will end up with a total shading value of 1.4, the surface will appear too bright and you’ll need to decrease diffuse to 0.6 in order to restore the total value to 1. In Mia material on the other hand, increasing reflections or refractions automatically decreases diffuse.
Because of its fast shading computations, Mia Material X is very effective for both 3D architectural visualizations and high quality 3D cartoon animations.
In this free interactive tutorial you find a complete list of all Mia material x attributes simulated with a dynamic value slider. This documentation is offered by the Swiss High-End rendering company VisualCenter.