To simplify things, everything that can be created in the admin builder is called Item.
An Item can be
- Container: admin page, metabox, user profile
- AJAX button
- Field: Checkbox, Code, Color, Date, EDD license, Enable, File, Gallery, Multicheck (custom or predefined datas), Number, Radio (custom or predefined datas), Select (custom or predefined datas), Sortable, Text, Textarea, Upload, WYSIWYG
Each item is described separately later in this tutorial