\newcommand vs \def for class development

Question

By reading the contents of the file book.cls (the standard book class for LaTeX), I can see there are many uses of \def and \newcommand. What I don't seem to understand is why one is used in some cases and why the other in other cases. I couldn't find a pattern.

I understand the differences between\def and \newcommand, and I am aware that I should prefer the latter in everyday document writing. However, my question goes on the context of class and package development. For example, I could define a \title command for my new book class, that also accepts a subtitle as optional argument, by writing

\def\doc@title{}
\def\doc@subtitle{}
\def\title{\kernel@ifnextchar[{\@@title}{\@title}%
\def\@title#1{
    \@@title[]{#1}%
}
\def\@@title[#1]#2{%
    \gdef\doc@subtitle{#1}
    \gdef\doc@title{#2}
}

However, I could also say

\newcommand\doc@title{}
\newcommand\doc@subtitle{}
\newcommand{\title}[2][]{
    \gdef\doc@subtitle{#1}
    \gdef\doc@title{#2}
}

Is there any reason to prefer one way over the other in the previous pieces of code?

Similarly, the book.cls file has this line:

\DeclareOption{10pt}{\renewcommand\@ptsize{0}}

Why is it not written as

\DeclareOption{10pt}{\def\@ptsize{0}}

In class/package development, what are the criteria to use \newcommand in some situation and \def in other?

Answer 1

Use expl3, and all the \XX_new:N command families and their nuances enter the picture too. :)

Expl3 is much easier to code in.

Same (philosophical) answer at base as the linked question, though.

MWE

% arara: lualatex
\documentclass{article}
\ExplSyntaxOn
\tl_new:N \g_ds_doctitle_tl
\tl_new:N \g_ds_docsubtitle_tl
\tex_def:D \mymeta #1 {
    \textsf{\textbf{ #1 }}
}
\tex_def:D \commandbuild #1#2#3 {
        \c_backslash_str
        #1
        \c_underscore_str
        #2
        \c_colon_str
        #3
}
\tex_def:D \texdef {
        \begin{center}
        The ~ difference ~ between ~ X ~ and ~ Y
        \end{center}
        \begin{center}
        \mymeta { \commandbuild {tex}{def}{D} } ~ and ~ all ~ the ~
        \mymeta { \commandbuild {XX}{new}{N} } ~ commands
        \end{center}
        }
\NewDocumentCommand { \mytitle } { o +m } {
    \tl_gset:Nx \g_ds_doctitle_tl { #2 }
    \IfValueT { #1 }
    {
            \tl_gset:Nx \g_ds_docsubtitle_tl { #1 }
        \regex_replace_all:nnN
                { (subtitle)+ }
                { \c{underline} \cB\{ \0 \cE\} }
                \g_ds_docsubtitle_tl

        \tl_gput_left:Nn \g_ds_docsubtitle_tl 
        { 
                \vspace { 2\tex_baselineskip:D }
                \begin{center}
        }
        \tl_gput_right:Nn \g_ds_docsubtitle_tl 
        { 
                \end{center}
        }

        \tl_gput_right:Nn \g_ds_doctitle_tl 
        { 
                \large \upshape 
        }
        \tl_gput_right:NV \g_ds_doctitle_tl \g_ds_docsubtitle_tl
}

\tl_gput_left:Nn \g_ds_doctitle_tl { \Large }
\tl_gput_right:Nn \g_ds_doctitle_tl { \texdef }


}
\NewDocumentCommand { \printtitle } { } {
        \tl_if_empty:NF \g_ds_doctitle_tl
        {
        \group_begin:
        \center
        \itshape
        \tl_use:N \g_ds_doctitle_tl
        \group_end:
        }
}
\ExplSyntaxOff
\mytitle[My subtitle A]{Multiparagraph
My Title A}
%============  document 
\begin{document}
\printtitle
\end{document}