N.B.: these are a feature for the more expert user, so learners and general users can skip straight to the next section, Document Settings, and return to this later if needs be.
Config files are a way of pre-setting some document preferences/attribute defaults so that all new documents receive that customisation. Effectively it allows customisation of a limited number the app's built-in defaults.
Next in the cascade are Document-level Settings…
See also—notes linking to here: