Struggling to understand why leaf nodes don't need wholes?

1

In the docs (section on schema, subsection Serialization and Parsing) it says

Leaf nodes don’t need a hole in their DOM representation, since they don’t have content.

I can’t clearly understand why it is so. Can someone please explain and provide an example where leaf nodes don’t have content?

2

They do not have content, so it is not necessary to specify where the content is rendered.