[S]ymmetries of gravitational systems, that is, changes of coordinates that can be applied to space-time and that leave invariant the large-distance behaviour of the gravitational field. They are known as asymptotic symmetries and can be thought of as a generalization of Poincaré symmetry for systems endowed with a weak gravitational field. In other words, these symmetries are those one would observe by looking at a gravitational system “from far away”B. Oblak, BMS Particles in Three Dimensions, Springer Theses