  We here present three characterizations of not necessarily causal, rational
functions which are (co)-isometric on the unit circle: (i) Through the
realization matrix of Schur stable systems. (ii) The Blaschke-Potapov product,
which is then employed to introduce an easy-to-use description of all these
functions with dimensions and McMillan degree as parameters. (iii) Through the
(not necessarily reducible) Matrix Fraction Description (MFD).
  In cases (ii) and (iii) the poles of the rational functions involved may be
anywhere in the complex plane, but the unit circle (including both zero and
infinity).
  A special attention is devoted to exploring the gap between the square and
rectangular cases.
