Setting this property sets the specified index and deselects any previously selected items. If you want to preserve the multiple selection state, use the getItemState or setItemState methods instead. Specifying an index value of -1 clears the list. Getting this property returns a value of -1 when no items are selected.
Reference_Syntax.selectedIndex
Specifying an index value of -1 clears the list. Specifying any other valid value results in only that item being selected.
Form Model

selectedIndex