作为起点:
\documentclass[tikz, margin=3mm]{standalone}
\begin{document}
\begin{tikzpicture}
\node (n1) [draw, minimum width=4em] {Bar};
\draw[<-, shorten <=1mm]
(n1.south west) -- ++ (-1,-0.5) node [below, align=center] {B\\ (note)};
\end{tikzpicture}
\end{document}