Hi,
When dragging from a ruler a line appears that can serve as a guide to install components on the PCB or for aligment. Dragging from the upper ruler for a new horizontal line. Dragging from the right ruler for a new vertical line. Attached a screen shot taken from GIMP how a guide line can be added from a ruler to help with the design and aligment. These lines can be deleted, moved (adjusting to grid), and can snap "things" to it.

Thanks.