% Cursor extras: gap, leftover rail, a tracked absolute node.
\documentclass[border=8pt]{standalone}
\usepackage[en]{superfig}

\sfsetrole{x}{sfTeal}
\sfsetrole{y}{sfViolet}

\begin{document}
\begin{tikzpicture}
\sfleftrail{4mm}
\sfrow{R1}{12mm}
  \sfnode[role=x]{A}{$A$}{12mm}{10mm}
  \sfgap{8mm}
  \sfnode[role=y]{B}{$B$}{12mm}{10mm}
\sfrowend
\sfnode[role=x, at={($(B.south)+(0,-14mm)$)}]{C}{$C$}{12mm}{10mm}
\sfarrow{B.south}{C.north}
\sflane{R1}
\sfcaption{A}{$A$}{rail}
\sfnolane
\sfcaptiontop{B}{after gap}
\sfcaption{C}{$C$}{absolute, tracked}
\sfbbox{all}
\sftopformula{F}{$C \leftarrow B$}
\end{tikzpicture}
\end{document}