Normal Map alpha channel

Im not quite sure what the normal map alpha channel is for on these .dds files. It looks like an inverted version of the rgb map. Im currently working on a magnum texture and i would appreciate it if you could tell me what it is for.

extra:
I have not exported yet, i came here after my search turned up no results.
If this post is incomplete let me know and ill do my best to fix it.

Do you even know what the normal maps do? If you know that then what you just said would make a lot more sense.

Yes i do know what normal maps do. I am a 3d modeler and a texture 'artist'. Quite frankly, i dont use .dds as i dont do hardcore importing into games and stuff. I use a regular bmp with a standard normal map. Not this heightmap style. I dont use alpha channels on my normal maps either, only on my diffuse really.

Just to clarify i do know what all these maps do.

Diffuse : Color
Normal : bump map
Spec : shinyness
Alpha : transparency


What i dont understand is, why does a normal map (height map) need an alpha channel (transparency) and since it obviously does, what does it do.