 Composite systems postulate (CSP):  A quantum system represented by \(S: (\mathcal{O}, H)\), with initial state \(\varrho_0 \in \mathcal{O}'\), is composite when it can be partitioned into two quantum systems \(S^1 : (\mathcal{O}^1, H^1)\) and \(S^2 : (\mathcal{O}^2, H^2)\) such that (i) \(\mathcal{O} = \mathcal{O}^1 \otimes \mathcal{O}^2\), and (ii) \(H = H^1 \otimes I^2 + I^1 \otimes H^2\) (where \(I^1\) and \(I^2\) are the identity operators in the corresponding tensor product spaces). In this case, we say that \(S^1\) and \(S^2\) are subsystems of the composite system \(S = S^1 \cup S^2\). If the system is not composite, it is elemental. 