Consider the code
\ExplSyntaxOn
% ... lots of stuff 1 ...
\ExplSyntaxOn
% ... lots of stuff 2 ...
\ExplSyntaxOff
% ... lots of stuff 3 ...
\ExplSyntaxOff
(a) How is stuff at positions 2 and 3 interpreted at the moment: as LaTeX2e code or as expl3 code?
(b) Is such code supposed to be interpreted at all, and, if so, how?
(An aside: such constructs started appearing in my files due to copy-and-paste.)
No, you can't. There's no stack implemented.
When \ExplSyntaxOn is encountered, some characters have their category code are changed, but their previous category code is stored in the definition of \ExplSyntaxOff (that does nothing, by default).
When \ExplSyntaxOff is next encountered, it resets the category codes to the previous states and redefines itself to do nothing.
Here's an example:
\documentclass{article}
\usepackage{expl3}
\ExplSyntaxOn
\tl_show:N \ExplSyntaxOff
\ExplSyntaxOn
\tl_show:N \ExplSyntaxOff
\ExplSyntaxOff
\tl_show:N \ExplSyntaxOff
\ExplSyntaxOff
\stop
And here's the output on the terminal: you see that the expl3 codes are not valid any longer after the first \ExplSyntaxOff and \tl_show:N raises an error.
> \ExplSyntaxOff=\char_set_catcode:nn {9}{10}\char_set_catcode:nn
{32}{10}\char_set_catcode:nn {34}{12}\char_set_catcode:nn
{38}{4}\char_set_catcode:nn {58}{12}\char_set_catcode:nn
{94}{7}\char_set_catcode:nn {95}{8}\char_set_catcode:nn
{124}{12}\char_set_catcode:nn {126}{13}\tex_endlinechar:D =13\scan_stop:
\bool_set_false:N \l__kernel_expl_bool \cs_set_protected:Npn \ExplSyntaxOff
{}.
<recently read> }
l.5 \tl_show:N \ExplSyntaxOff
?
> \ExplSyntaxOff=\char_set_catcode:nn {9}{10}\char_set_catcode:nn
{32}{10}\char_set_catcode:nn {34}{12}\char_set_catcode:nn
{38}{4}\char_set_catcode:nn {58}{12}\char_set_catcode:nn
{94}{7}\char_set_catcode:nn {95}{8}\char_set_catcode:nn
{124}{12}\char_set_catcode:nn {126}{13}\tex_endlinechar:D =13\scan_stop:
\bool_set_false:N \l__kernel_expl_bool \cs_set_protected:Npn \ExplSyntaxOff
{}.
<recently read> }
l.8 \tl_show:N \ExplSyntaxOff
?
! Undefined control sequence.
l.11 \tl
_show:N \ExplSyntaxOff
?
\ExplSyntaxOn and \ExplSyntaxOff are switches that change certain category codes to allow for a different coding syntax. They're synonymous to the \makeatletter...\makeatother pairs used to allow @'s in macros.
A showcase of something similar using font switches may be conclusive:
\documentclass{article}
\begin{document}
\bfseries
First % ... lots of stuff 1 ...
\bfseries
Second % ... lots of stuff 2 ...
\mdseries
Third % ... lots of stuff 3 ...
\mdseries
Last % ... lots of stuff last ...
\end{document}
One could consider \mdseries reverting what \bfseries has done, but that doesn't imply they are nestable.
Switches are not the same as grouping, which provides a scope as well as nesting.
\ExplSyntaxOff, the state is reverted to normal category codes. Just copy and paste the part between them – egreg Nov 11 '16 at 18:40