By default, the calculation of the Brightness feature (Feature View > Object Features > Layer values > Mean > Brightness) is the sum of all input layers divided by the number of input layers per object. In the example below, we have 4 input layers (Layer 1, Layer 2, Layer 3 & Layer 4), therefore the brightness for each object will be ((Layer 1 + Layer 2 + Layer 3 + Layer 4) / 4).

Only layers which only have positive values are included. This means, that input layers with negative values won't be included in the Brightness feature calculation. If all input layers do not have positive-only values, then there are no channels available for brightness calculation and the resulting feature value is (undefined).

If you only want to have Layer 1 and Layer 3 included in the calculation of the Brightness feature, you can easily do so via the Classification menu (Classification > Advanced Settings > Select Image Layers for Brightness...). In the Define Brightness dialog, simply select the layers you want to include in the Brightness feature calculation (highlighted in blue) and click "OK". You can select multiple layers by holding down CTRL. In the image below you see different examples or the Brightness feature calculation based on difference selections, on the left Layer 1 & Layer 2 are selected for the calculation of the Brightness feature, in the center Layer 3 & Layer 4 are selected and on the right all input Layers are selected. Enjoy showing off with this little trick to your colleagues.