App Form Fields
Form fields in an App Form, for the most part, work the same way as form fields would work in a regular form created in the Forms app. The difference is that the some form fields are automatically generated for an App form based on what attributes are assigned to an item, and cannot be deleted. This is to keep the structure of data between items created in the administration and items created in the public site as similar as possible.
If the attributes for your items ever change, the form will need to be updated by simply editing and re-saving the form. This will "refresh" the fields to use the new/updated attributes.