a form validation issue

I checked the system/libraries/Form_validation.php file(CI 3.1.0), I was wondering is it because of the _prepare_rules function? So, the callback function and rule's order was change.
It says ""Callbacks" are given the highest priority (always called), followed by 'required' (called if callbacks didn't fail), and then every next rule depends on the previous one passing.
So, the "callback" function will be run first, then "required" rule and others?

Sorry, I'm not a native English speaker, maybe I misunderstand the meaning.