\usepackage{booktabs}          % preamble

\begin{tabular}{lccc}
 \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}