  Constraints on spin observables coming from discrete symmetries such as P, C,
T and identical particles may be divided in two types: 1) classical ones, which
insure the invariance of the cross sections under the symmetry operation; 2)
non-classical ones, which can only be obtained at the level of amplitudes.
Similarly, positivity constraints can be divided into classical and
non-classical constraints. The former insure the positivity of the cross
section for arbitrary individual polarisations of the external particles, the
latter extend this requirement to the case of entangled external spins. The
domain of classical positivity is shown to be dual to the domain of
separability
