Descendant pages are pages that have the same or a higher level than the current page in the page system.
The descendant's pages can be queried to create an illustrative list in index pages.
You can query them by using the descendants table of the page sql.
Example:
select from descendants where is_index = 1 and depth = 1 order by name asc
If you take this query and use it:
<iterator>
<data path=":docs:docs">select from descendants where is_index = 1 and depth = 1 order by path asc limit 10 </data>
<fragment>
* [[$path|$name]] (Level: $level)
</fragment>
</iterator>
you would get the second level index page (we have limited to 10 with the limit 10 clause).