\usepackage{booktabs}          %preamble
\usepackage{multirow}          %preamble    

\renewcommand{\multirowsetup}{\centering}
\begin{tabular}{cccc}
 \toprule
  Case & Box A & Box B & Remarks \\ \midrule
  \multirow{2}{10mm}{A}
  & $+X$ &     $-1.0$  & \\ \cmidrule(lr){2-3}
  & $-X$ &     $-7.9$  & \\
 \bottomrule
\end{tabular}