Extended multiselect usually acts like a single-selection list box, so when you choose an item, you cancel all other selections. When you hold down Shift while choosing the mouse or pressing an arrow key, you select items sequentially from the current item. When you hold down Ctrl while choosing the mouse, you add single items to the list.

Single select allows only one item at a time to be selected.

Simple multiselect toggles the selection on an item in the list when it's chosen with the mouse or the Spacebar is pressed when the focus is on the item.