In trying to answer this question, I thought it would be easy to add a new float environment using the float package. However, the labeling of the caption is not getting correctly formatted.
Because I'm creating a new float that I want labeled with a side caption, I dug into the sidecap package and created a new environment to match the new float.
Here's a MWE:
\documentclass{book}
%% setting up new float
\usepackage{float}
\floatname{mySpecialFloat}{Special Float}
\newfloat{mySpecialFloat}{tbhp}{lst}[chapter]
\newcommand{\listofSpecialFloats}{\listof{mySpecialFloat}{List of Special Floats}}
%% setting up side captions
\usepackage[wide]{sidecap}
\makeatletter
\@ifdefinable\SC@specialFloat@vpos{\def\SC@specialFloat@vpos{b}}
\newenvironment{SCspecialFloat}{\SC@float[\SC@specialFloat@vpos]{mySpecialFloat}}{\endSC@float}
\newenvironment{SCspecialFloat*}{\SC@dblfloat[\SC@specialFloat@vpos]{mySpecialFloat}}{\endSC@dblfloat}
\makeatother
%% setting up formatting for cpations
\usepackage
[
font=footnotesize,
format=plain,
labelfont={bf,sf},
textfont={it},
width=10pt
]{caption}
\usepackage{lipsum}
\pagestyle{empty}
\begin{document}
\listofSpecialFloats
\begin{mySpecialFloat}
\centering
Hello
\caption[toc entry for hello]{Hello}
\end{mySpecialFloat}
\begin{SCspecialFloat}
\begin{minipage}[t]{\textwidth}
\lipsum[1-2]
\end{minipage}
\caption[toc entry for lipsum]{My caption content}
\label{fig:src:01}
\end{SCspecialFloat}
\end{document}
which produces

What I find interesting is that everything seems to be working correctly. The listing command knows about all my special floats. The only thing that doesn't seem to be working correctly is the label for the caption for the side caption float.
I know this could be accomplished with newfloat, but I'm wondering what's happening here. Why is the label name getting lost in the sidecap environment?
UPDATE
There seems to be a conflict in naming conventions used by float and those expected by sidecap.
It seems this can be fixed if I add the following line to my code for setting up the captions:
\@namedef{mySpecialFloatname}{Special Float}