  We give an overview of literature related to J\"urgen Ehlers' pioneering 1981
paper on Frame theory--a theoretical framework for the unification of General
Relativity and the equations of classical Newtonian gravitation. This
unification encompasses the convergence of one-parametric families of
four-dimensional solutions of Einstein's equations of General Relativity to a
solution of equations of a Newtonian theory if the inverse of a causality
constant goes to zero. As such the corresponding light cones open up and become
space-like hypersurfaces of constant absolute time on which Newtonian solutions
are found as a limit of the Einsteinian ones. It is explained what it means to
not consider the `standard-textbook' Newtonian theory of gravitation as a
complete theory unlike Einstein's theory of gravitation. In fact, Ehlers' Frame
theory brings to light a modern viewpoint in which the `standard' equations of
a self-gravitating Newtonian fluid are Maxwell-type equations. The consequences
of Frame theory are presented for Newtonian cosmological dust matter expressed
via the spatially projected electric part of the Weyl tensor, and for the
formulation of characteristic quasi-Newtonian initial data on the light cone of
a Bondi-Sachs metric.
