Sticky Selection, also known as "Checkbox Mode" is a selection style known from many apps with multiple selection lists. The effect is pretty much identical to holding the CTRL key while you click on an item or hit the space key on the focused item: The previous selections are not touched and the current item's selection state toggles. Also clicking on the white space will not unselect the selected items.

Sticky Selection is obviously very practical when you work on long lists and want to pick out particular items manually: You won't easily lose your work by the slip of a finger.

Last not least there's a toolbar button for quick toggling: