ExtendedSelect only has effect when the MultiSelect is true.  

When ExtendedSelect is true, the Shift key (in conjunction with the left mouse button, or arrow keys) is used to select a contiguous sequence of list items. And the Ctrl key is used (in conjunction with the left mouse buttton) to select non-contiguous list items.  

When ExtendedSelect is false, Multi-Select operates very differently. The Shift and Ctrl keys are not recognised. List items are always selected individually in any sequence. Reselecting a selected list item will deselect it. Use the arrow and spacebar keys, or the left mouse button to make selections. 

MultiSelect