If not given, cross references render with the .html extension as in:
<a href=not-readme.html#h2-in-not-the-readme>
This way, those links will work when rendering locally to .html files which is the default behaviour of:
ourbigbook .
If given however, the links render without the .html as in:
<a href=not-readme#h2-in-not-the-readme>
which is what is needed for servers such as GitHub Pages, which automatically remove the .html extension from paths.
This option is automatically implied when publishing to targets that remove the .html extension such as GitHub pages.