Yes, it’s documented on this page: https://www.advancedcustomfields.com/resources/get_field/. Maybe it’s not specific enough because it’s designed to be working with other field types too.
Basically, ACF saves the ID of an object (post, page, user, taxonomy, etc.) in the database. If you set the format value (the third parameter) to
false, it will get the raw entries in the database instead of parsing it to an object.
I hope this makes sense 🙂