%%
%% The LaTeX Companion, 3ed
%%
%% Example 16-7-39 on page II-567 in "Changing format directives".
%%
%% 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{111.0pt}
%
  \setlength\parindent{0pt}
%  Use biber program

%StartShownPreambleCommands
\usepackage{biblatex}  \addbibresource{tlc-ex.bib}
\DeclareListFormat{mylistformat}
  {Location \thelistcount/\theliststop: #1%
    \ifnumgreater{\value{listcount}}{\value{liststop}-1}%
                 {!}%     -- list end reached
                 {,\\}%   -- in list
  }
%StopShownPreambleCommands

\begin{document}
\citelist{location-list}[mylistformat]{location}
  \\[3pt]
\citelist{location-list}{location} % default directive
\end{document}