Guide Lines are horizontal, vertical, or slanted lines that can be placed anywhere in the work area window to help with object placement and nodes alignment.
To create a new guide line, position cursor on either vertical or horizontal ruler, depress the left mouse button and drag cursor to the work area.
Guide lines can be used for polygon and polyline splitting. Place the guide line on the object. Select object and then select the guide line. Click right mouse button on the guide line to access pop-up menu. Choose Slice Selected Objects command.