  The kinetic theory recently implemented in heavy ion reactions combines a
non-local and non-instant picture of binary collisions with quasiparticle
features. We show that the non-instant description is compatible with the
spectral concept of quasiparticles while the commonly used variational concept
is consistent only with instant collisions. The rearrangement energy, by which
the variational concept surpasses the spectral one, is shown to be covered by a
medium effect on non-instant collisions.
