The unformatted summary style is only available for summary styles, which are when an argument has been set to provide a summary if it was not provided with a value. This summary provides the possible candidates for the argument one after another with no special formatting. If inline is selected, the summary items will be enclosed within <span> tags. Otherwise the items will be in <div> tags.
You can also elect to display the number of matching records for the argument, plus change the number of items per page for the summary. This is often useful because summary views are often quite small, but other views quite space intensive. It is very common to have far more records available in the summary view than in the more normal view.