ExtendedSelect works with the MultiSelect property. If ExtendedSelect is true and MultiSelect is true, the user can select an item then hold down the Shift key and select another and all the items in between the two selected items also become selected.  

If the user doesn't hold down the Shift or Ctrl key while selecting a second item, the first selected item becomes unselected— in other words, the user must use the Ctrl key to select multiple noncontiguous items, or the Shift key to select a range of items. 

If ExtendedSelect is false and MultiSelect is true, the user can select multiple items without using the Shift or Ctrl key, but they can't select a range of items in one operation. 

If MultiSelect is false, the setting of ExtendedSelect has no effect as the user will not be able to select more than one item at a time in the list box. 

MultiSelect