Questions on WordPress Codex Concept

Adding here.

register_post_type function have a parameter public which controls how the type is visible to authors and readers.

If public parameter of a post type is true then post type will be exclude_from_search, publicly_queryable, show_in_nav_menus, and show_ui. Read more about it in register_post_type function.

Also all post types must be registered to use in WordPress. So there is not such thing as un-registered post type but a registered post type can have option to be public or not based on what value you supply.