This mode is used to link two points in both of the Edit Windows.
The Control Windows title is initially set to L1.
The Mouse Pointer is set to L1.
A point can then be selected in either of the windows by clicking the mouse.
The Control Windows title is then set to L2.
The Mouse Pointer is set to L2.
Another point can then be selected in either of the windows by clicking the mouse. The points will be joined by a line.
Note: this line is to enable points to be identified more easily. It is not used by the rendering process.