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To answer the original question, the Input Output Reference description for Output:Variable Site Daylighting Model Sky Clearness does not agree with the source code. The reported variable is SkyClearness which is calculated as:

    Zeta = 1.041 * pow_3( SunZenith );
    SkyClearness = ( ( DifSolarRad + BeamSolarRad ) / ( DifSolarRad + 0.0001 ) + Zeta ) / ( 1.0 + Zeta );

This is used in two ways. First it is used to calculate the sky luminous efficacy. This calculation uses binned coefficients

The other use of SkyClearness is to determine the sky luminance at each time step. From the Engineering Reference

Time-Step Sky Luminance The sky luminance distribution, ψ, for a particular time step is expressed as a linear interpolation of two of the four standard skies — ψcs, ψts , ψis and ψos — described above under “Sky Luminance Distributions.” The two sky types that are interpolated depend on the value of the sky clearness.

Here there are three ranges for SkyClearness that are meaningful SkyClearness <1.2 uses the overcast and intermediate sky luminances 1.2 < SkyClearness <= 3 uses the intermediate and clear turbid sky luminances SkyClearness > 3 uses the clear turbid and clear sky luminances.

To answer the original question, the Input Output Reference description for Output:Variable Site Daylighting Model Sky Clearness does not agree with the source code. The reported variable is SkyClearness which is calculated as:

    Zeta = 1.041 * pow_3( SunZenith );
    SkyClearness = ( ( DifSolarRad + BeamSolarRad ) / ( DifSolarRad + 0.0001 ) + Zeta ) / ( 1.0 + Zeta );

This is used in two ways. First it is used to calculate the sky luminous efficacy. This calculation uses binned empirical coefficients which have 8 ranges, the last for SkyClearness > 6.2 (thus >6 is a clear sky).

The other use of SkyClearness is to determine the sky luminance at each time step. From the Engineering Reference

Time-Step Sky Luminance The sky luminance distribution, ψ, for a particular time step is expressed as a linear interpolation of two of the four standard skies — ψcs, ψts , ψis and ψos — described above under “Sky Luminance Distributions.” The two sky types that are interpolated depend on the value of the sky clearness.

Here there are three ranges for SkyClearness that are meaningful SkyClearness <1.2 uses the overcast and intermediate sky luminances 1.2 < SkyClearness <= 3 uses the intermediate and clear turbid sky luminances SkyClearness > 3 uses the clear turbid and clear sky luminances.

To answer the original question, Output:Variable Site Daylighting Model Sky Clearness is calculated as:

    Zeta = 1.041 * pow_3( SunZenith );
    SkyClearness = ( ( DifSolarRad + BeamSolarRad ) / ( DifSolarRad + 0.0001 ) + Zeta ) / ( 1.0 + Zeta );

This is used in two ways. First it is used to calculate the sky luminous efficacy. This calculation uses binned empirical coefficients which have 8 ranges, the last for SkyClearness > 6.2 (thus >6 is a clear sky).

The other use of SkyClearness is to determine the sky luminance at each time step. From the Engineering Reference

Time-Step Sky Luminance The sky luminance distribution, ψ, for a particular time step is expressed as a linear interpolation of two of the four standard skies — ψcs, ψts , ψis and ψos — described above under “Sky Luminance Distributions.” The two sky types that are interpolated depend on the value of the sky clearness.

Here there are three ranges for SkyClearness that are meaningful SkyClearness <1.2 uses the overcast and intermediate sky luminances luminances

1.2 < SkyClearness <= 3 uses the intermediate and clear turbid sky luminances luminances

SkyClearness > 3 uses the clear turbid and clear sky luminances.

To answer the original question, Output:Variable Site Daylighting Model Sky Clearness is calculated as:

    Zeta = 1.041 * pow_3( SunZenith );
    SkyClearness = ( ( DifSolarRad + BeamSolarRad ) / ( DifSolarRad + 0.0001 ) + Zeta ) / ( 1.0 + Zeta );

This is used in two ways. First it is used to calculate the sky luminous efficacy. This calculation uses binned empirical coefficients which have 8 ranges, the last for SkyClearness > 6.2 (thus >6 is a clear sky).

The other use of SkyClearness is to determine the sky luminance at each time step. From the Engineering Reference

Time-Step Sky Luminance The sky luminance distribution, ψ, for a particular time step is expressed as a linear interpolation of two of the four standard skies — ψcs, ψts , ψis and ψos — described above under “Sky Luminance Distributions.” The two sky types that are interpolated depend on the value of the sky clearness.

Here there are three ranges for SkyClearness that are meaningful meaningful:

SkyClearness <1.2 uses the overcast and intermediate sky luminances

1.2 < SkyClearness <= 3 uses the intermediate and clear turbid sky luminances

SkyClearness > 3 uses the clear turbid and clear sky luminances.