What does the number on a sail mean? Eg: GP9463
The number on the sail is a distinctive unique identifier, which helps you know which boat is which. This is similar to football, where all the players have a number on their t-shirt.
The sail number also represents the number of boats in that class and every single one is different (there are never any boats that are the same design and have the same sail number).
You would use your sail number when racing, so that the committee boat or finish boat can identify which boat finished where.
Hope that answers your question, but do not hesitate to ask further questions if you have any.