There must always be at least one Sub Header in a form in order to place other Form elements, the Sub Header allows for a clean sectioning within the form. For Example: separating contact details from job details. These can be deleted, but anything that is within the sub Header section will also be removed.

Sub Headers can only be added at the end of a section, so if you have existing form elements or fields that you'd like placed inside a new Sub Header section, then it's best to add the section first, and then drag the fields you want in the new section into there.

Having sections separated by Sub Headers also allows for simple repeating of the section, if required.