It selects or unselects vertices, lines, and faces within the area dragged by a mouse. While dragging a mouse, you can select again with the shift key and partly unselect with the Ctrl key.
When you press the [Rect] or the [Rope] button of 'Edit option' group in command panel, you can select within a rectangle or freehand line by dragging a mouse while using [Select] command or [Move] command without [Rect] or [Rope] command.