A Special Mark, as defined by the International Association of Lighthouse Authorities, is a sea mark used in maritime pilotage to indicate the boundary of an obstruction, administrative area such as a speed limit, water skiing or mooring area, or to highlight other features such as outfall sewerage pipes.
It is recognisable by its yellow colour and X top-mark.
Its distinctive sequence of flashing light consists of one quick flash with intervals of 5 seconds.