The difference between the first and the second input raw biases (in ADU) is computed. The population standard deviation of the pixel values of this difference, divided by the square root of 2, is taken as the RON.

More information can be found at the <a
href="http://www.eso.org/observing/dfo/quality/XSHOOTER/qc/bias_qc1.html">plot tutorial page</a>