Modified command to obtain an alternative list of figures

EDIT

This is an answer that I think meets the OP's requirements as he stated in his comment to my earlier communication.

% mlofprob.tex  SE 597456   EDITED VERSION WORKS !!!!!!!!!!!!!!!!!!!!!!!!

documentclass{article}
usepackage{graphicx}
usepackage{caption}
usepackage{subcaption}
usepackage{duckuments}

usepackage{comment}

captionsetup[subfigure]{list=true}

usepackage{tocloft}
newcommand{listmfiguresname}{M List of Figures}
newlistof{extracmd}{mlf}{listmfiguresname}

newcommand{extracommand}[2]{%
  addtocontents{mlf}{#1: Figure #2par}
}

begin{document}

listoffigures
listofextracmd

begin{figure}
centering
includegraphics{example-image-a}
%label{fig:alef}
caption[Alef figure.]{Alef.}
label{fig:alef}
extracommand{example-image-a}{ref{fig:alef}}
end{figure}

begin{figure}
    begin{subfigure}[]{textwidth}
    centering
    includegraphics[width=textwidth]{example-image-b}
 %   label{fig:bet}
    caption[Bet figure.]{Bet.}
    label{fig:bet}
    extracommand{example-image-b}{ref{fig:bet}}
    end{subfigure}
    
    begin{subfigure}[]{textwidth}
    centering
    includegraphics[width=textwidth]{example-image-c}
%    label{fig:gimel}
    caption[Gimel figure.]{Dalet.}
    label{fig:gimel}
    extracommand{example-image-c}{ref{fig:gimel}}
    end{subfigure}

    caption[Bet + gimel figure.]{Bet + gimel.}
end{figure}

end{document}

** ORIGINAL POSTING**

Not an answer but it shows what I tried. After your above comment I did not understand the basics of what you are after. I tried for a new `listof...` but after some failures I gave up (although I'm sure that it is possible) ---time constraints came into this. 

I thought that perhaps what you needed was just to list the figure numbers. Below I have done that by using `ref{<fig-label>}`. I had to modify your original code as `label` has to come after `caption`.

 

        % mlofprob.tex  SE 597456   DIDN'T WORK!!!!!!!!!!!!!!!!!!!!!!!!
    
    documentclass{article}
    usepackage{graphicx}
    usepackage{caption}
    usepackage{subcaption}
    usepackage{duckuments}
    
    usepackage{comment}
    
    captionsetup[subfigure]{list=true}
    
    begin{comment}
    usepackage{tocloft}
    newcommand{listmfiguresname}{M List of Figures}
    newlistof{extracmd}{mlf}{listmfiguresname}
    
    end{comment}
    
    providecommand{extracommand}[1]{%
      addcontentsline{mlf}{}{#1}par
    }
    renewcommand{extracommand}[1]{}
    
    
    begin{document}
    
    listoffigures
    %mlistoffigures %command to be defined
    %listofextracmd
    %
    %Desired output of mlistoffigures (using extracommand{}):
    % 1. Fig. 1
    % 2. Fig. 2
    %    a Fig. 2a
    %    b Fig. 2b
    
    textbf{Figure numbers}
    
    ref{fig:alef}
    
    ref{fig:bet}
    
    ref{fig:gimel}
    
    
    begin{figure}
    centering
    includegraphics{example-image-a}
    %label{fig:alef}
    caption[Alef figure.]{Alef.}
    label{fig:alef}
    extracommand{Fig. 1} %command to be defined
    end{figure}
    
    begin{figure}
        begin{subfigure}[]{textwidth}
        centering
        includegraphics[width=textwidth]{example-image-b}
     %   label{fig:bet}
        caption[Bet figure.]{Bet.}
        label{fig:bet}
        extracommand{Fig. 2a}
        end{subfigure}
        
        begin{subfigure}[]{textwidth}
        centering
        includegraphics[width=textwidth]{example-image-c}
    %    label{fig:gimel}
        caption[Gimel figure.]{Dalet.}
        label{fig:gimel}
        extracommand{Fig. 2b}
        end{subfigure}
    
        caption[Bet + gimel figure.]{Bet + gimel.}
        extracommand{Fig. 2}
    end{figure}
    
    end{document}