  In the dawn of the multi-messenger era including gravitational waves, which
was marked by the first ever coincident detection of gravitational waves and
electromagnetic radiation it is important to lay back and think about
established knowledge. Numerical simulations of binary neutron star mergers and
simulations of short GRB jets have to combine efforts in order to understand
such complicated and phenomenologicaly rich explosions. We review the status of
numerical relativity simulations with respect to any jet or magnetized outflow
produced after merger. We compare what is known from such simulations, with
what is used and obtained from short GRB jet simulations propagating through
the BNS ejecta. We point out facts that are established and can be considered
known, and things that need to be further revised and/or clarified.
