  We review the density matrix formalism and the positivity conditions for
general multiple spin asymmetries, taking as an example the case antiproton +
proton -> antiLambda + Lambda, in which one, two or three spins are analyzed.
Some aspects related to quantum information and entangled states are discussed.
Some positivity domains for pairs and triplets of spin parameters are
displayed, together with the experimental points. The case of inclusive
reaction is also treated, taking as an example the spin- and transverse
momentum- dependent quark distributions.
