Details

The viewer not only allows studying the symmetry of specific planes, but also visualizes the density of the atoms in certain directions. This is important for a process called channeling (see the Wikipedia entry). In this case a (high-energy) atom or ion penetrates the crystal. If the atoms of the crystal have a low density in the direction of penetration, the penetration depth is probably high, and vice versa. One may say that in low-density directions channels open for the penetrating particle. Usually, these channels open in high-symmetry crystal directions. To visualize the effect of different cross sections, it is possible to change the size.

References:

For more structures that appear in crystals of pure elements see N. W. Ashcroft and N. D. Mermin, Solid State Physics, Philadelphia: Sounders College Publishing, 1976.

C. Kittel, Introduction to Solid State Physics, 7th ed., New York: John Wiley & Sons, Inc., 1996.