This mixer module has four inputs. If the fourth input isn't patched then this channel carries the signal of the internal noise generator which is made entirely with analog transistor technology. This configuration if practical if you'd like to use the module in a subtractive synthesis setup as an oscillator mixer before the filter.
In addition there is a dedicated noise output, this way the signal can be used for other tasks.
noise output, mix output
3U Eurorack module, 6HP wide