\documentclass[../OpenAppliedMusicTheory.tex]{subfiles}

\externaldocument{../OpenAppliedMusicTheory}
\externaldocument{checkpoints}
\externaldocument{chapter100} %chapter number for reference here


\begin{document}
    
    \sfchapter{Template}{23}
    This is a template subfile! :)

    This will be added to as things get more complex.

\end{document}