For example, if you want 20nm wide lines you should use a spot size no larger than about 5nm. According to this plot, you could choose the 200um aperture at 3 nA, or you could choose the 300um aperture at 1 nA. So the 200 um aperture is the better choice. If you set up an exposure job that calls for different currents and different apertures, the system will switch to them automatically. Yes: it will even switch apertures automatically!