%%
%% The LaTeX Companion, 3ed
%%
%% Example 8-4-24 on page I-627 in "Numbering newly defined color boxes".
%%
%% 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{159.0pt}
% small headings for the example
\makeatletter
\renewcommand\section{\@startsection{section}{1}{\z@}%
                                    {-1.25ex \@plus-1ex \@minus-.2ex}%
                                    {1.5ex}%
                                    {\normalfont\normalsize\bfseries}}
\makeatother

  \setcounter{section}{4}
%use spotcolor mix
\IfPackageLoadedTF{colorspace}
  {\AtBeginDocument{%
      \renewtcolorbox[number within=section,
              number format=\alph]{exabox}[3][]
             {colback=blue!5,colframe=spotblue!75!spotblack,
              fonttitle=\bfseries,label=#3,nameref=#2,
              title=Exa~\thetcbcounter: #2,#1}}}
  {}

%StartShownPreambleCommands
\usepackage{nameref,tcolorbox}
\newtcolorbox[auto counter,number within=section,
              number format=\alph]{exabox}[3][]
             {colback=blue!5!white,colframe=blue!75!black,
              fonttitle=\bfseries,label=#3,nameref=#2,
              title=Exa~\thetcbcounter: #2,#1}
%StopShownPreambleCommands

\begin{document}
\section{A Section}
\begin{exabox}[sharp corners=south]{Title text}{exa:A}
  This exhibits a numbered tcolorbox.
\end{exabox}
The example on page \pageref{exa:A} is numbered
\ref{exa:A}. Its title is ``\nameref{exa:A}''.
\end{document}