I need to understand something about coordinate systems. according to haushoffer in message 3 in https://www.physicsforums.com/threads/diffeomorphism-invariance-in-gr.485023/ "The main point is that with a fixed background, you can shift fields with respect to that background. That fixed background defines points which have a physical meaning, and this can be covered by different coordinate systems. So coordinates do not have physical meaning. Without fixed background, as in GR, you don't have this. If you transform all the physical fields, you also tranform the metric. Points loose their meaning and only distances are physically meaningful." Actually I need an example. When it's fixed background. How come it can be covered by different coordinate system, and what does it mean. Please give something with fixed background that can be covered by different coordinate system. For example. My computer table is fixed. So what different coordinate system can cover it? And if the background is not fixed like in classical GR. Does it mean it can't be covered by different coordinate system? and why is that?