Usually the Alpha is luminance is when a texture has no alpha channel so by checking that ,Mental Ray will use instead the black and white values to generate
a transparency mask...that is in a nutshell, I get lost with this one sometimes.
File nodes have alpha luminance off by default.
You'll find it in some other important nodes such as displacement and Layer textures, just from the top of my head.
No, Specular maps, reflection maps, glossy maps, bump...none of these deal with transparency so alpha is luminance would have no effect.
You can use a texture mask to control the areas of reflection or transparency and that's when Alpha comes into place.
Controlling the highlights in a material with a map means making it shinier or more dull with black, greys and white tones in the texture map to achieve the desired look.
I chose the Glossy Reflection because in that simple sphere I needed to convey worn rusted areas so it needed to be glossy/blurred ...unlike the specular layer that would give you just a less reflective area but the reflection itself still remains sharp.
Shaders can get very complex once you start aiming for photo realism, over time you'll learn all the nuances....until they introduce a new shader again.
