  In this second paper we complete the classical description of an isolated
system of "charged positive-energy particles, with Grassmann-valued electric
charges and mutual Coulomb interaction, plus a transverse electro-magnetic
field" in the rest-frame instant form of dynamics.
  In particular we show how to determine a collective variable associated with
the internal 3-center of mass on the instantaneous 3-spaces, to be eliminated
with the constraints ${\vec {\cal K}}_{(int)} \approx 0$. Here ${\vec {\cal
K}}_{(int)}$ is the Lorentz boost generator in the unfaithful internal
realization of the Poincare' group and its vanishing is the gauge fixing to the
rest-frame conditions ${\vec {\cal P}}_{(int)} \approx 0$. We show how to find
this collective variable for the following isolated systems: a) charged
particles with a Coulomb plus Darwin mutual interaction; b) transverse
radiation field; c) charged particles with a mutual Coulomb interaction plus a
transverse electro-magnetic field.
  Then we define the Dixon multipolar expansion for the open particle
subsystem. We also define the relativistic electric dipole approximation of
atomic physics in the rest-frame instant form and we find the a possible
relativistic generalization of the electric dipole representation.
