Special Marks are yellow buoys with a yellow cross on top. The yellow cross is an ‘X’ shape instead of a plus sign.
Special marks can mean anything not covered by the rest of the buoys. Their meaning can be looked up in almanacs, pilot books and in some cases on charts.
Examples of what they can mean are:
The light for a special mark can be any rhythm other than that reserved for a white light. For example, an East Cardinal buoy flashes three white lights. Therefore, a yellow special mark will never flash three white lights.