If given, the toplevel output of each input source is always non-split, and a split version is not generated at all.
This of course overrides the \H splitDefault argument for toplevel headers, making any links go to the non split version, as we won't have a split version at all in this case.
  "h": {
    "splitDefault": true,
    "splitDefaultNoToplevel": true,
= My first header

== My second header
When converted with:
ourbigbook --split-headers my-first-header.bigb
would lead only to two output files:
  • my-first-header: not split
  • my-second-header: split
Without splitDefaultNoToplevel we would instead have:
  • my-first-header: split
  • my-first-header-nosplit: not split
  • my-second-header: split
The initial use case for this was in OurBigBook Web. If we didn't do this, then there would be two versions of every article at the toplevel of a file: split and nosplit.
This would be confusing for users, who would e.g. see two new articles on the article index every time they create a new one.
It would also mean that metadata such as comments would be visible in two separate locations.
So instead of filtering the duplicate articles on every index, we just don't generate them in the first place.