The term refers to sections that have a parent/child relationship via either of the:
rather than via the usual header hierarchy.
Secondary children show up for example on the tagged metadata section, but not on the table of contents, which is what the header hierarchy already shows.
Secondary children are normally basically used as "tags": a header such as Bat can be a direct child of Mammal, and a secondary child of Flying animal, or vice versa. Both Mammal and Flying animal are then basically ancestors. But we have to chose one main ancestor as "the parent", and other secondary ancestors will be seen as tags.
This option first does ID target from title conversion on the argument, so you can e.g. keep any spaces or use capitalization in the title as in:
= Animal

== Flying animal
{child=Big bat}

== Big bat
TODO the fact that this transformation is done currently makes it impossible to use "non-standard IDs" that contain spaces or uppercase letters. If someone ever wants that, we could maybe add a separate argument that does not do the expansion e.g.:
= Animal

== Flying animal
{childId=Big bat}

== Big bat
{id=Big bat}
but definitely the most important use case is having easier to type and read source with the standard IDs.

Ancestors