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.

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    $\begingroup$ The diffeomorphism corresponds to Lagrangian particle mappings, which are volume preserving, expressing conservation of mass. We also have a particle relabeling symmetry in fluid mechanics which expresses vorticity conservation. The vector field here is simply the velocity of the fluid. $\endgroup$ – Nick P Jul 6 '19 at 0:40

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