Well I need to correct me after trying more things.

Alternate_Diffuse seems to just create RGB values according to it's input. If I just put a Simple Color node with let's say RGB pink into it, then the surface will be pink. It does not care about light.
If I put a Diffuse node instead of Simple Color into it, then the surface will be pink also, but the Diffuse node also takes care about the influence of the light (if a light is switched on).

So I think I got it.

The Poser Surface Diffuse does everything for me. The Alternate Diffuse gives me the opportunity to do everything myself.

Isao, I think that's what you said, right?