%%
%% The LaTeX Companion, 3ed
%%
%% Example 7-4-7 on page I-548 in "Continuing captions across floats".
%%
%% 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{109.80112pt}

  \setcounter{page}{1}

  % for the book examples we shorten the vertical spaces
  \makeatletter
  \renewcommand\section{\@startsection{section}{1}{\z@}%
    {-1.25ex \@plus-1ex \@minus-.2ex}%
    {1ex}%
    {\normalfont\Large\bfseries}}
\renewcommand\section{\@startsection{section}{1}{\z@}%
                                    {-1.25ex \@plus-1ex \@minus-.2ex}%
                                    {.5ex}%
                                    {\normalfont\large\bfseries}}
  \makeatother

  \setlength\footskip{16pt}
  \pagestyle{origplain} % plain is disabled by the class

  \setlength\textheight{5.4cm}
  \setlength\intextsep{8pt}

  \setlength\parindent{8pt}  % shorten this for the example output

  \usepackage{microtype}     % improve line breaking

%StartShownPreambleCommands
\usepackage[labelfont={bf}]{caption}
\renewcommand\theContinuedFloat
             {\alph{ContinuedFloat}}
%StopShownPreambleCommands

\begin{document}
\listoffigures \bigskip
\begin{figure}[!ht]
 \ContinuedFloat* % <- star form
 \centering\fbox{Figure I}
 \caption{First figure}\label{FI}
\end{figure}
\begin{figure}[!ht]
 \ContinuedFloat
 \centering\fbox{Figure II}
 \caption{Continuation}\label{FII}
\end{figure}
Figures \ref{FI} and \ref{FII} in
this example are subnumbered, while
Figure~\ref{FIII} is not. \par Use
\verb=\ContinuedFloat*= if the first
float should be labeled 1a already.
\begin{figure}
 \centering\fbox{\LARGE Figure III}
 \caption{Third figure}\label{FIII}
\end{figure}
\end{document}