In these buoys, that are employed to sign a danger to navigation, the word "cardinal" refers to the cardinal points of the compass North, South, East and West. Thus, a cardinal buoy marks the passage to the safest and deepest water. For instance, a West cardinal buoy like the one on the picture shows that the safest water is found West of it (and not that the danger is to the West).
Cardinal buoys are marked with letters and not with numbers. Cardinal buoys are painted with yellow and black horizontal bands; the pattern of colours on the buoy determine whether it is a North cardinal buoy, a South cardinal buoy, an East cardinal buoy or a West cardinal buoy. The two black cones on the top indicate which cardinal buoy is. This one have the cones pointing each other - West. If the cones had their bases facing each other - East. If the cones are both pointing up - North and if the cones are both facing down - South.
The light, which is always white, has its characteristics according to the cardinal mark and is easily remembered by making an analogy with a clock (1 flash - North; 3 flashes - East; 6 flashes - South and 9 flashes West).