In order to access this option you should select the EXPERT OPTIONS sheet (close to the upper left corner of the main program window).
The ACCURACY parameter controls the numerical accuracy of most (not all) integration routines inside the CasP code. Higher values lead to less certain results, but speed up the program. Values below 1E-5 do not improve the numerical results and values above 1E-2 may lead to unpredictable uncertainties for some target shells.
The relative uncertainty of the integrated total stopping cross sections is roughly in the order of ACCURACY, but there is no linear scaling between the required ACCURACY and the resulting accuracy. For heavy targets, for high velocities, and for low accuracy values a large number of grid points is chosen in the automatic impact-parameter selection. This gives a proper picture of the electronic energy loss due to the inner shells close to the nucleus as well as for the long-ranged dipole contributions of valence electrons.
If you observe spikes in the GRAPHICS output for Q_e(b), this is an indication for an insufficient numerical accuracy. In this case, please lower the ACCURACY value and repeat the calculation (this might give you some time to drink a cup of coffee during the computation).