Sections do not provide intrinsic information logic, but allow building a logical structure in the form and are used to build the table of contents (ToC) of the form.
To add you first section, click on the button Add Element at the bottom of the DDE and selecting Type 'section'. Fill out the label and click on Add.
You can add sections to your own liking.
To add subsections, click on the triangle on the left of the section. This opens up the section and an additional Add Element button is shown to add new elements, such as sections, within the opened section.
To reorder sections (and in a later step any other content element), simply drag & drop the element to the place you want.