Hi Oberon,
The Div fields are available to allow you to create your own custom HTML Div tags. Particuarlly useful if you are using the minima template.
An example for use, create a Div Start field, and define the behavior as Start. Place that field in the beginning of your fields (top) of the Content Type. Then create a Div End and define that behavior as End, and place that field a the bottom. So the result is that the Div is "wrapping" the fields that you want for that particular style element. Finally in the Markup Class (#3) option define the style element for the div (span9).
Hope that helps,
Randy