%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-2-2 on page I-36 in "Numbering headings".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{171.0pt}
%StartShownPreambleCommands
\renewcommand\thesubsection
    {\fbox{\thesection.\arabic{subsection}}}
%StopShownPreambleCommands

\begin{document}
\setcounter{section}{3}
\subsection{A mistake}\label{wrong}
Referencing a subsection in this format produces
a funny result as we can see looking at
subsection~\ref{wrong}. We get a boxed reference.
\end{document}