Question How is unity_IndirectSpecColor calculated?

It appears to be missing from the documentation, I have created a ticket to get this sorted out. Sorry for the inconvenience.

The shader variable is the constant indirect specular color, only used when sampling of the environment map is disabled and set based on RenderSettings state. It is a black float4 if you don't have a valid skybox material.

If your skybox material is valid, it is calculated like this:

Code (CSharp):

1. // Use constant part of skybox ambient SH as indirect specular color when no environment map is used
2. // Don't apply ambient intensity
3. const SphericalHarmonicsL2& sh = GetAmbientProbe();
4. const ColorRGBAf c(sh.GetCoefficient(SphericalHarmonics::kColorChannelRed, 0),
5.                    sh.GetCoefficient(SphericalHarmonics::kColorChannelGreen, 0),
6.                    sh.GetCoefficient(SphericalHarmonics::kColorChannelBlue, 0));
7. unity_IndirectSpecularColor = LinearToActiveColorSpace(c * m_ReflectionIntensity);