I don't think you understand what you're talking about. the realms being infinite absolutely matters. If you have two realms A and B. Both A and B are 3D spatially. Then both have coordinates A[x1, y1, z1] and B[x2, y2, z2]. If even one of these realms is spatially infinite, let's say A is infinite, then the coordinates x1, y1, and z1, will occupy an infinite number of points in the 3D space. If that is so, then unless the container separates A and B using an extra dimension, A and B would overlap, making them not separate realms. But because they are separate, and one of them is infinite, and they are both contained by DW, DW needs to separate them both using a 4th spatial axis, mathematically speaking.