The distributions shown are: a carboxyl O- atoms around Arg sidechains, b amino NH3 atoms around Glu sidechains, and c backbone O atoms around Phe sidechains.
The illustrations on the left-hand side show the atoms making up the distributions, with the contours picking out the favourable regions where clustering occurs. The distributions and contours are projected back, and down, onto the three walls of the box to give a clearer picture of the 3D arrangement.
The illustrations on the right-hand side show just the central sidechain and the contours defining the most favoured regions.