It is stated that in the holographic principle (e.g., in http://en.wikipedia.org/wiki/Holographic_principle) that the the description of a volume of space is encoded on a light-like boundary to the region. But with respect to which position in the volume? In a black hole, it is clear, because all the light from all the positions inside the black hole do not go beyond its event horizon. But in another volume, the light cone of point A inside the volume is not the same as the light cone of point B ≠ A inside the volume. So I do not quite understand the boundary meant here. Thanks for any enlightenment.