What is a reference frame in General Relativity?

Abstract: In General Relativity, reference frames must be distinguished from coordinates. The former represent physical systems interacting with the gravitational system, aside from possible approximations, while the latter are mathematical artefacts. We propose a novel three-fold distinction between Idealised Reference Frames, Dynamical Reference Frames and Real Reference Frames. In doing so, the paper not only clarifies the physical significance of reference frames, but also sheds light on the similarities between idealised reference frames and coordinates. It also analyses the salience of reference frames to define local Dirac observables and to propose a physical interpretation to diffeomorphism gauge symmetry.