The fact that general relativity is a diffeomorphism invariant theory means that there is no preferred co-ordinate system in GR. How is it possible to understand this in the context of relativistic hydrodynamics?
Also, while talking about diffeomorphisms generated by a vector field. Which are the vector fields, in the hydrodynamic sense, that generate these diffeomorphisms?
I know that this arbitrary diffeomorphism invariance leads to the stress-energy Tensor conservation equation, but am not able to make complete sense of what diffeomorphism invariance actually means.