Create floating subsection

Question

I am writing a summary mostly including equations but also some pictures. Now I would like to place the subsections including these pictures floating, such that the arrange themselves nicely.

I know how to place floating pictures. But is it possible to do the same to a whole subsection?

Thanks for your answers!

Edit: The floating section works. Altough in combination with {multicol} the newfloat{subsec} is no longer displayed.

\documentclass{article}
\usepackage{lipsum,caption,graphicx,float,titlesec}
\usepackage{multicol} %multiple columns
\newfloat{subsec}{h}{lop}

\begin{document}
\begin{multicols*}{3}
\tableofcontents
\listoffigures
\section{The Section}
Here is my non-floating introduction, which I place here just to 
demonstrate the margins.

\begin{subsec}[ht]
\subsection{Veni}
\lipsum[1-2]
\end{subsec}

\end{multicols*}
\end{document}

Answer 1

REVISED ANSWER

With the original answer (floated subsections and margin notes), I was unable to get multicols to encapsulate floats, and I don't see how that is possible. So, while the OP speaks of floats within columns, this revised answer takes a slightly different interpretation.

If the OP's goal is for subsections to remain as intact blocks, then one alternative to floating them would be to put them each in their own minipage, which will not break across columns. That is the approach I adopt here, but it also puts an added constraint on the OP to keep subsections under column in length.

My \mpar alternative to \marginpar is adopted here, as well, but adds an optional argument l or r to indicate which column the call is made from. In theory, 3 or more columns could be handled with a further rewrite.

So here is the result of this MWE:

\documentclass{article}
\usepackage{lipsum,caption,graphicx,tabto,multicol}
\newenvironment{subsec}[1][]{\par\noindent\begin{minipage}{5.9cm}}{\end{minipage}}
\def\mmargin{20pt}
\def\mwid{3cm}
\def\mposl{\dimexpr-\mmargin-\mwid}
\def\mposr{\dimexpr\textwidth+\mmargin}
\newcommand\mpar[2][l]{\tabto*{\csname mpos#1\endcsname}\smash{\parbox[t]{\mwid}{%
  \raggedright#2}}\tabto*{\TabPrevPos}}
\begin{document}
\tableofcontents
\listoffigures
\clearpage
\begin{multicols}{2}
\section{The Section}
 Here is my non-floating introduction, which I place here just to 
  demonstrate the margins.
\begin{subsec}[ht]
\subsection{Veni}
\lipsum[1]
Margin note call \mpar{Here is my longish margin note}
\lipsum[4]
\end{subsec}
\begin{subsec}[ht]
\subsection{Vidi}
\lipsum[3]
Margin note call \mpar[r]{Here is my second longish margin note}
\lipsum[4]
\end{subsec}
\begin{subsec}[ht]
\subsection{Vici}
\lipsum[4-5]
\begin{center}
\includegraphics[width=2in]{example-image-A}
\captionof{figure}{My Figure}
\end{center}
\end{subsec}
\end{multicols}
\end{document}

ORIGINAL ANSWER:

Perhaps something like this... Here I create a new float, subsec, using the answer described at Floating an algorithm?. Note that you cannot then place floating figures inside of the subsections, but can nonetheless get them there with \includegraphics and \captionof techniques, as I show on the final float.

As I mentioned in my comment to the OP's question, LaTeX floats must fit on a single page.

I have verified that it works when inserting \tableofcontents and \listoffigures at the top of the document.

Based on comment by the OP, EDITED to provide faux margin note \mpar capability (using the tabto package), by specifying \mpos for the location relative to the left margin of the text, and \mwid for the margin-note width. I just set the margin note as normal text, but \mpar can be easily adjusted for font size and style.

\documentclass{article}
\usepackage{lipsum,caption,graphicx,float,tabto}
\newfloat{subsec}{h}{lop}
\def\mpos{\dimexpr\textwidth+20pt}
\def\mwid{3cm}
\def\mpar#1{\tabto*{\mpos}\smash{\parbox[t]{\mwid}{%
  \raggedright#1}}\tabto*{\TabPrevPos}}
\begin{document}
%\tableofcontents
%\listoffigures
\section{The Section}
 Here is my non-floating introduction, which I place here just to 
  demonstrate the margins.
\begin{subsec}[ht]
\subsection{Veni}
\lipsum[1]
Margin note call \mpar{Here is my longish margin note}
\lipsum[2]
\end{subsec}
\begin{subsec}[ht]
\subsection{Vidi}
\lipsum[3]
\end{subsec}
\begin{subsec}[ht]
\subsection{Vici}
\lipsum[4-7]
\begin{center}
\includegraphics[width=2in]{example-image-A}
\captionof{figure}{My Figure}
\end{center}
\end{subsec}
\end{document}