When to check Alpha is Luminosity?

[3Dlunatic]

I work with Maya and Substance Painter, using the Metallic Roughness workflow. When I import maps from SP, I generally use 4 types in my aiStandardSurface shader: Base Color, Metallic, Roughness, and Normal. (Sometimes also Emissive). When I plug in my maps, I never know when I should check in Alpha is Luminosity as I don't know what it means and can't find help on it. For a second part of this question, I use sRGB for the color mode for Base Color, and Raw for everything else. Am I doing it right?

[Stephen.Blair]

If a texture has no alpha channel, then you use Alpha is Luminosity to get an alpha channel (that you can use with opacity)