list 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 a list of possible candidates for the argument in a standard HTML list. Like the normal list style, you may set this list to be ordered or not.
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.