  In early days of quantum theory it was believed that the results of
measurements performed on two distant physical systems should be uncorrelated
thus their quantum state should be separable it means described by a simple
tensor product of the individual local state vectors or a tensor product of
individual local density operators. It was shown many years ago by EPR that two
systems which interacted in the past and separated afterwards had to be
described in most cases by particular non-separable states which are called
entangled. It was noticed by Zanardi et al. that a Hilbert space of possible
state vectors of compound physical system could be partitioned in different way
by introducing various tensor product structures induced by the experimentally
accessible observables (interactions and measurements). Therefore a separable
state in one partition could become entangled in different partition. In this
sense the entanglement was relative to a particular set of experimental
capabilities. Continuing this line of thought Torre et al. claimed to prove
that for any separable state there existed strongly correlated physical
observables therefore all quantum states were entangled. The same claims one
may find in several recent papers. In this paper we will discuss in operational
way the differences existing between separable, non-separable and entangled
states and we will show that the conclusions of Torre et al. were unjustified.
A sufficient condition for entanglement is the violation of BI-CHSH and/or
steering inequalities. Since there exist experiments outside the quantum
physics violating these inequalities therefore in the operational approach one
cannot say that the entanglement is an exclusive quantum phenomenon.
