The \gls*{cls} (\texttt{.cls}) defines the page layout, heading
styles and various \glspl{command} and \glspl{environment} needed for a
particular type of document.  The class file is specified using the
command
\begin{definition}
\glsentrytext{documentclass}\oarg{\meta{options}}\marg{\meta{class-name}}
\end{definition}%
where \meta{class-name} is the name of the
file without the \texttt{.cls} extension.  All \LaTeX\ documents
must start with this command. This book uses the
\icls{scrbook} class.
