The Code Zero is a cross between a genoa and an asymmetrical spinnaker that is used for sailing close to the wind in light air. Code Zero was initially an attempt to circumvent a rating rule by making a large genoa for close reaching on boats that were measured with non-overlapping genaos. The Code Zero got around the rule by measuring in as a very narrow-flat spinnaker with shape similar to a reaching geona.  When not constrained by rating rules, cruising sailors have a lot more options on the size and shape of a “code” sail. 

https://www.uksailmakers.com/sails-overview-cruising-spinnakers/cruising-code-zero