Code 0 was a term that came about to identify a sail that was outside the "normal" sail wardrobe. For example big boats classify sails G or J 1,2,3,4,5 and A Kites as A 1,2,3,4. The J or G 1 being your light air jib or genoa and the A1 being your VMG kite (less cloth in the shoulders smaller girths etc).

These reaching sails that try to beat the design rules as a high reaching sail generally get slapped with a Code0 or A0 designation. For example the TP 52 had a huge genoa that they classified as a Code 0 until the class made them legal for distance racing, which at that time they became known as Masthead Genoas.

Anywhere else in the world the RED sail above would be classified as a A0.