Have section numbering use specific strings

Question

I am trying to create a document outlining a work breakdown structure. I would like the first section to be numbered "R3" instead of with the number one, the second section to be numbered "R1" instead of the number two, and the third section to be numbered "SPF" instead of the number three.

Beyond the third section, it would be nice if subsequent sections could be numbered starting from four. Subsections, subsubsections, paragraphs, etc., should have the usual arabic numbering.

For example, the 3rd subsubsection of the 1st subsection of the 2nd section would be: "R1.1.3 Rebuild of the Alu chambers"

Can this be done?

(I know this is the point at which I should share what I have tried, but apart from some fumbling around with \fnsymbol I am entirely lost, and am not even sure what to search for.)

Answer 1

Just use several \renewcommand instructions to modify the appearance of the \thesection macro.

\documentclass{article}
\begin{document}

%%% First section. 
\renewcommand\thesection{R3}
\section{Sec1}
\subsection{Sec1 Subsec1}
\subsubsection{Sec1 Subsec1 Subsubsec1}
\subsubsection{Sec1 Subsec1 Subsubsec2}
\subsubsection{Sec1 Subsec1 Subsubsec3}

%% Second section.
\renewcommand\thesection{R1}
\section{Sec2}
\subsection{Sec2 Subsec1}
\subsubsection{Sec2 Subsec1 Subsubsec1}
\subsubsection{Sec2 Subsec1 Subsubsec2}
\subsubsection{Rebuild of the Alu chambers}

%% Third section.
\renewcommand\thesection{SPF} % why not...
\section{Sec3}
\subsection{Sec3 Subsec1}
\subsubsection{Sec3 Subsec1 Subsubsec1}
\subsubsection{Sec3 Subsec1 Subsubsec2}
\subsubsection{Sec3 Subsec1 Subsubsec3}

%% Fourth section and beyond
\renewcommand\thesection{\arabic{section}} % back to standard setup
\section{Sec4}
\subsection{Sec4 Subsec1}
\subsubsection{Sec4 Subsec1 Subsubsec1}
\subsubsection{Sec4 Subsec1 Subsubsec2}
\subsubsection{Sec4 Subsec1 Subsubsec3}

\section{Sec5}
\subsection{Sec5 Subsec1}
\subsubsection{Sec5 Subsec1 Subsubsec1}
\subsubsection{Sec5 Subsec1 Subsubsec2}
\subsubsection{Sec5 Subsec1 Subsubsec3}
\end{document}