This setting determines how much of the harmonic distortion component that is to be included in time domain.
30 % as shown above is used as default in the standard setups. If this is increased, a longer sized window is used on the distortion components. If a longer window is used, more noise will be included, but if a too short window is used, the whole distortion impulse response may not be included. This can be inspected by plotting the impulse response on a logarithmic vertical axis.
It is also possible to calculate the effective decay range of
each of the harmonic distortion components from the Calculate menu. In order to
do this, the time window must not be set manually and must be set to include the
harmonic distortion component to be evaluated. This is set in the dialog box
opened by clicking the 
button in Plot->Calculation Options….