To select more than one item with the mouse, hold down the shift key while making the second and subsequent selections with the mouse. Note that alternatively you can use the Ctrl key for multiple selection by setting a Preference setting (see Preferences - Application Settings).
To clear the selection of one item in a set of multiply selected items hold down the shift key and select it again.
To clear the selection of all selected items in a window, click the mouse in the window anywhere but on top of a selectable item (without holding the shift key down.
Multiple-select in Tables follows the Windows standard of shift-click is an extend select (all objects between first selected item and clicked on item are selected) and ctrl-click is a specific extend select (the item clicked on it added to the list of items selected).