  We discuss the status of thermal model descriptions of particle ratios in
central nucleus-nucleus collisions at ultra-relativistic energy. An alternative
to the
  ``Cleymans-Redlich'' interpretation of the freeze-out trajectory is given in
terms of the total baryon density. Emphasis is placed on the relation between
the chemical equilibration parameters and the QCD phase boundary. Furthermore,
we trace the essential difference between thermal model analyses of data from
collisions between elementary particles and from heavy ion collisions as due to
a transition from local strangeness conservation to percolation of strangeness
over large volumes, as occurs naturally in a deconfined medium.
  We also discuss predictions of the thermal model for composite particle
production.
