[xeonmc]

Any 3D shape/distribution can be expressed as some infinite mixture of Cartesian x-y-z (plane) waves, but if your object is closer to being radially symmetric then it might be more appropriate to express it as a mixture of azimuth-declination & radius waves instead. Technically you can choose any esoteric shape to split of your three degrees-of-freedom and your description will be mathematically identical as long as your DoF basis don’t have redundant parts, but usually we tend to use either Cartesian or spherical descriptions, and the frequency-domain (reciprocal space) description of those choice of symmetry corresponds to [xyz plane waves] or [spherical harmonics (angular part) + Spherical Hankel functions (radius part)]