Why is the measured area of a parent sail required to be marked on smaller sails?

It is useful for race committees and competitor sailors to be re-assured that reduced size sails used by a competitor comply with his boat’s certificate. Prior to 2016 the class rules required reduced size sails to ‘fit inside’ the profile of the largest (measured) sails.

Although this was an apparently simple rule it was impossible for sails genuinely made to the same measured dimensions to comply with the rule if their luff curve (or leech length) differed from the largest (measured) sail. No marks identifying any sail with any boat were required.

The 2016 class rules require each sail to comply with the boat’s certificate. This is straightforward to establish using the sail measurement system and works for sails that are of maximum size but have a different luff curve. To show that reduced size sails have been checked against a boat’s certificate and have been found to comply, the measurer is required to add the area of the parent sail to reduced size sails.

Please see the realted Q&A

Can a sail be used on more than one boat where each boat has different parent sail areas?