What's Flatten and How can I use it?

REST API
1

In the responses from a call to /_schema, I can see some flatten-related fields. I don’t see anywhere to turn this on in the configuration page. What do these options do? Can I call them on a URL like /_data/my_table/?related=*&flatten=true or something similar?

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Is there documentation for flatten and flatten_drop_prefix somewhere?

Thanks!
Josiah