This output format is mostly a joke.
It is an intermediate step in automatic ID from title, that unlike HTML output does not have any tags.
So for example, converting:
with the id output format produces simply:
instead of the HTML output:
This conversion type is useful in situations that users don't expect conversion to produce any HTML tags. For example, you could create a header:
= My \i[asdf]
and then following the automatic ID from title algorithm, that header would have the more commonly desired ID my-asdf, and not my-<i>asdf</i>.
Besides being more intuitive, this also guarantees greater format portability, in case we ever decide to support other output formats!
We decided to expose this format from the CLI just for fun, as it posed no extra work at all as it is treated internally exactly like any other conversion format.
And it also was a good start generalizing OurBigBook to multiple outputs, as this is a simple format.
The conversion is very simplistic, it basically just pastes the content of most macros. Important exceptions to that include:
So for example if we had:
= Title with more

\Image[my.png]{title=My \i[nice] \x[title]
then the image ID would be image-my-nice-title:
  • image- automatically added prefix
  • my : literal text
  • nice: extract the content of the italic
  • title: note how we use the href title instead of what is actually rendered, Title with more, because \x is treated somewhat specially for performance reasons
Stuff that is primarily non-textual like images is just completely removed. We could put effort in outputting their title correctly, but meh, not worth the effort.