<p>This replaces your standard custom post meta form. Allows extension of page, post and custom post type by allowing you to create customised post form fields when adding or editing post types.</p>
<p>This is typically for developers to extend your typical post and post type form. You can use get_post_meta function in your theme template  to display the saved data.</p>


WP Extra Fields

Allows extension of page, post and custom post type by allowing you to create customised form fields to include when adding or editing post types.

WP Extra Fields vs Customize Posts