This option is analogous to \H parent argument, but for includes.
For example, consider you have:
= Animal

== Dog

== Cat

== Bat
and now you want to split Cat to cat.bigb.
If you wrote:
= Animal

== Dog

\Include[cat]

== Bat
Cat would be a child of Dog, since that is the previous header, which is not what we want.
Instead, we want to write:
= Animal

== Dog

\Include[cat]{parent=animal}

== Bat
and now Cat will be a child of Animal as desired.

Ancestors