3D spheres, cylinders and cones can be made 'solid' to ensure that the field inside the electrode is exactly zero. Also, the space-charge inside a 'solid' electrode is zero.
An electrode can also be made 'hollow', to cancel the 'solid' attribute of any electrode that surrounds it. The combination of the 'solid and 'hollow' attributes enables a volume such as a thick wall to be 'solid'.
Note that these options are essentially 'cosmetic, in the sense that they can improve the appearance of contour plots but do not have any affect on ray trajectories (the program switches the option off when rays are being traced).
The results might be indeterminate if more that one electrode is made 'solid'.
The volumes are those of the original electrodes, so any inscribing expansions are not taken into account.