%%
%% The LaTeX Companion, 3ed
%%
%% Example 6-7-16 on page I-486 in "siunitx --- Scientific numbers in tables".
%%
%% 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{183.0pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
  \usepackage{siunitx}
%StopShownPreambleCommands

\begin{document}
  \begin{tabular}{|S|S[table-alignment-mode=none,
                       table-text-alignment=left]|
                   S[table-alignment-mode=none,
                     table-alignment=right]|}
   {Default} & {Left}    & {Right}  \\
      12.345 & 12.345      & 12.345   \\
         6.7 & 6.7         & 6.7      \\
    -88.8(9) & -88.8(9)    & -88.8(9) \\
       4.5e3 & 4.5e3       & 4.5e3
  \end{tabular}
\end{document}