  The majority of current understanding of the quantum correlations is in the
field of non-relativistic quantum mechanics. To develop quantum information and
computation tasks fully, one must inevitably take into account the relativistic
effects. In this regard, the spin is one of the central tools. For this
purpose, it is of paramount importance to understand and characterize fully the
theory of spin in relativistic quantum information theory where the spin states
act as qubit. This area is still far from being resolved. As a result, this
article will explore the recent studies of the concepts of the spin and spin
quantum correlations in inertial frames and some apparent paradoxes regarding
this concept. We will mainly focus on the problem of characterizing the spin,
reduced spin density matrices and spin quantum correlations in inertial
reference frames and the apparent paradoxes involved therein. Another important
aspect is the use of tools of quantum field theory to extend several concepts
in non-relativistic domain to relativistic one. In this regard, we analyze the
development of the theory of relativistic secret sharing and a correlation
measure namely the entanglement of purification.
