The paper by B.S. Cirel'son explores the quantum generalizations of Bell's inequality, focusing on the limitations of quantum correlations. Despite violating Bell's inequality, quantum correlations satisfy weaker inequalities. The author proves specific inequalities and discusses the more general case of instruments located in different space-time regions. The paper introduces four theorems, with proofs to be published elsewhere, and a brief discussion on their physical implications. The first theorem characterizes all possible quantum correlations between observables $A_k$ and $B_l$ with spectra in $[-1, +1]$. It shows that for $m=n=2$, the operators can be chosen as $2 \times 2$ matrices representing spin components of a spin one-half particle, and the inequality $c_{11} + c_{12} + c_{21} - c_{22} \leq 2\sqrt{2}$ holds for arbitrary quantum observables. This inequality is the greatest possible value for the considered linear combination of spin correlations.The paper by B.S. Cirel'son explores the quantum generalizations of Bell's inequality, focusing on the limitations of quantum correlations. Despite violating Bell's inequality, quantum correlations satisfy weaker inequalities. The author proves specific inequalities and discusses the more general case of instruments located in different space-time regions. The paper introduces four theorems, with proofs to be published elsewhere, and a brief discussion on their physical implications. The first theorem characterizes all possible quantum correlations between observables $A_k$ and $B_l$ with spectra in $[-1, +1]$. It shows that for $m=n=2$, the operators can be chosen as $2 \times 2$ matrices representing spin components of a spin one-half particle, and the inequality $c_{11} + c_{12} + c_{21} - c_{22} \leq 2\sqrt{2}$ holds for arbitrary quantum observables. This inequality is the greatest possible value for the considered linear combination of spin correlations.