  The problem of spectra formation in hydrodynamic approach to A+A collisions
is discussed. It is analyzed in terms of the two different objects:
distribution and emission functions. We show that though the process of
particle liberation, described by the emission function, is, usually,
continuous in time, the observable spectra can be also expressed by means of
the Landau/Cooper-Frye prescription. We argue that such an approximate duality
results from some symmetry properties that systems in A+A collisions reach to
the end of hydrodynamic evolution and reduction of the collision rate at post
hydrodynamic stage
