%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-2-3 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
\usepackage{titlesec}
\titlelabel{\fbox{\thetitle}\hspace{0.5em}}
%StopShownPreambleCommands

\begin{document}
\section{This is correct}\label{sec:OK}
Referencing a section using this definition
generates the correct result for the section
reference~\ref{sec:OK}.
\end{document}