Special mark

A Special Mark, as defined by the International Association of Lighthouse Authorities, is a sea mark used in maritime pilotage. It is recognisable by its yellow colour and X, (also referred to as a St. Andrews Cross or saltire) top-mark. It has a distinctive sequence of various flashes that does not match any other navigational mark flashes in its vicinity.

Purpose
Special marks can indicate:
 * Administrative areas
 * Water skiing areas
 * Anchorage areas
 * Mooring areas
 * Waiting areas
 * Marine farms
 * Oil wells
 * Dead ends
 * Pipelines
 * Spoil ground (an area where dredged material is deposited)
 * Historic wrecks
 * Protected areas
 * Outfall pipes (such as Stormwater, and Cooling water)
 * Sewerage pipes
 * Intake pipes
 * Submarine cables

Other uses

 * Buoys, such as a Weather buoy or Mooring buoy are coloured yellow or have a yellow light to indicate it is not an aid to navigation.