How does 'structure' work in an indexer?

https://devdocs.magento.com/guides/v2.3/extension-dev-guide/indexing-custom.html

Description:

There apparently is a <structure> element within an indexer.xml, but this is not described on the documentation page.

Possible solutions:

Explain what structure does optionally with a code example. A use case when to use db_schema.xml for an index or when to use structure would be helpful.