I would like to manually set subsections as letter or numbers in the same document
1. First section
1.P. First subsection
1.P.1. First subsubsection
1.P.2. Second subsubsection
1.V. Second subsection
2. Second section
2.1 First subsection
(Spacing not needed)
I would still like the numbering on section og subsubsection level to follow the usual counting rules.
Is it possible to do something like:
\section{First section}
\lettersubsection[P]{First subsection}
\subsubsection{First subsubsection}
\subsubsection{Second subsubsection}
\lettersubsection[V]{Second subsection}
\section{First section}
\subsection{First subsection}
But how to define \lettersubsection[]{}? Here I'm stuck. Is it even possible to combine counters like that? I think I can avoid mixing letters and numbers within the same section, if that can make things easier. Thank you very much!

Undefined control sequence. \NewCommandCopyI'm doing something very wrong, aren't I? – nunatak Oct 04 '21 at 19:32xparse, it will probably work. Can @egreg (or someone else) please confirm if that is the case? – enkorvaks Oct 05 '21 at 04:07\usepackage{letltxmacro}and\LetLtxMacro{\originalsubsection}{\subsection}. – egreg Oct 05 '21 at 06:52#3needed to be#4, see fix – egreg Oct 06 '21 at 06:55