Reference priority is a technique for specifying the relative front-to-back order in which 2D references appear in a 2D model when they are displayed in a view. Essentially, Display Priority adds a "calculated Z" value for these co-planar references. Display Priority is only useful or necessary for 2D models, since all elements in 3D models have their own explicit 3D coordinate space. In 3D models, elements closer to the eye are always drawn in front of elements further from the eye so display priority is not necessary.