The Page Layout Template Type is the very top level template that the Form Builder look at first. In all of the Template Sets available in a Form Builder installation, it contains the same content:
All three includes (header, page and footer) are required for this template type to function properly. If you omit any of them, that section will be omitted altogether.
Technically, you could move the Header or Footer Template Type contents into this template, but I personally liked keeping them separate just for clarity's sake.