While experimenting with different ways of presenting some parts of LaTeX document I created a table of contents which looks as a table. See the illustration below:

This was possible with a package called etoc.

A preamble, nothing special here:

\documentclass{article}

\usepackage{hyperref}

\usepackage{booktabs}
\usepackage{colortbl}
\usepackage{nicematrix}

\usepackage{etoc}

etoc allows to fully control how to typeset a ToC and even allows to specify which elements to display in the ToC. Let's say we introduce a custom sectioning level which we want to display as ToC. Somewhere in preamble we write:

% dummy sectioning level
\etocsetlevel{specialsection}{90}

Then we define a command which draws the ToC for us. We have to put it inside “group”:

\newcommand\mylistofsomething{%
  \begingroup

 % body follows below

  \endgroup
}

The following calls instruct etoc to handle only the section level we want:

  \etocglobaldefs% mandatory
  \etocsetnexttocdepth{4}

  % mimics subsubsection
  \etocsetlevel{specialsection}{4}

  \etocsetlevel{chapter}{99} % no chapters
  \etocsetlevel{section}{99} % no sections
  \etocsetlevel{subsection}{99} % no subsections
  \etocsetlevel{subsubsection}{99} % no subsections
  \etocsetlevel{part}{99} % no parts

For our custom section we set a style which in fact creates a table row for each entry:

  \etocsetstyle{specialsection}
  {}
  {\\}
  {
    \rowcolor{gray}
    \etocnumber&\etocname&just text%
  }

What remains is to set style for the whole ToC and write it. In ToC style definition we tell etoc to create a table:

  \etocsettocstyle{%
    \hypersetup{hidelinks}
    \begin{NiceTabularX}{\textwidth}{X[1,c]X[3,l]X[1,c]}[hvlines]
      \rowcolor{black}
      {\textcolor{white}{\textbf{number}}} &
      {\textcolor{white}{\textbf{name}}} &
      {\textcolor{white}{\textbf{text}}}
    }{
    \end{NiceTabularX}
  }

  \tableofcontents%

The defined command can be places as usual in the document body:

\mylistofsomething%

Here is how we can fill our custom ToC:

\section{section 1}

\subsection{subsection 1}

\subsubsection{subsubsection 1}

\etoctoccontentsline{specialsection}{\protect\numberline{somenumber1} name 1}

\section{section 2}

\subsection{subsection 2}

\subsubsection{subsubsection 2}

\etoctoccontentsline{specialsection}{\protect\numberline{somenumber2} name 2}

\etoctoccontentsline operator ads an entry to the ToC.

Special quirk \protect\numberline{somenumber2} can be used (this is optionally) to set an entity which can later be referenced as \etocnumer when filling the table.

name 1 or name 2 strings can be referenced as \etocname.

That's it!

So what is the problem? Quite basic:

• Initialize a list of something.
• Fill the list with elements (at arbitrary place in document).
• Reference an element of the list by numerical index.

Using LaTeX3 notation we can solve it in the following way.

At first, we define methods in the preamble to create a list, append an element to list and display an element from list:

\ExplSyntaxOn%
\NewDocumentCommand{\definesomethinglist}{}
{
  \clist_clear_new:N \l_something_clist
}
\NewDocumentCommand{\appendtosomethinglist}{m}
{
  \clist_put_right:Nn \l_something_clist { #1 }
}
\NewDocumentCommand{\getsomethinglistitem}{m}{%
  \textbf{\clist_item:Nn \l_something_clist { #1 }}
}
\ExplSyntaxOff%

Then we initialize a list before begin{document}:

\definesomethinglist%

Now we can append elements to the list somewhere in the document body:

\appendtosomethinglist{a note of interest}%

Displaying an element of interest:

\getsomethinglistitem{1}

Works for me. :–)