The Dropdown binding implements a css skinable replacement for the traditional html select element. It adds autocompleting capability as well as the conventional arrow button for maximum usability.
In its default operation it hides the select element and uses its existing items to generate the data for the autocompleter
In this mode an optional observable can be bound to the dropdown itself to reflect the selected option on your viewmodel
The other option is to leverage Knockouts existing "options" to provide the data for the select. In this mode, if the observable array that is bound to options is updated the dropdown control will also reflect the change
In this mode the standard "value" binding should be used to relect the change back to the viewmodel