I can't figure out what it's doing, but whatever it is, it doesn't make sense to me. Here's a simple example:

Shader "Dot3 Primary" {

Properties {_MainTex ("Texture", 2D) = ""}

SubShader{Pass
{
    SetTexture[_MainTex] {Combine primary Dot3 primary, primary}
    SetTexture[nothing] {Combine previous alpha}
}}

}

Why do I see the alpha channel of the texture instead of pure black? Also, why, if I use an actual Color for primary, does something else happen? (It's some kind of multiplication or something.)