This mode is used to unlink two linked points in both of the Edit Windows.
The Control Windows title is initially set to U1.
The Mouse Pointer is set to U1.
A point can then be selected in either of the windows by clicking the mouse.
The Control Windows title is then set to U2.
The Mouse Pointer is set to U2.
Another point can then be selected in either of the windows by clicking the mouse. If the points are currently joined then the line will be removed.