Wow - resurrecting an olde thread!
Anyhow, sail numbers are much like race car numbers - a convenience in starting and timing a race. They're up to the owner as long as they are unique and the boat's not in some high-level competition. If you're going to be entering your boat in a serious (Olympics, Nationals, Volvo round-the-world, etc) you need a unique number and US Sailing is the administrator (
https://www.ussailing.org/competition/offshore/services/#sail-numbers ). They'll register you with a sequential (5 digits now?) number and you can print USA ###### on everything you own, whether on sail, t-shirt, back tattoo, whatever. Some of the serious amateurs that I've talked to really like these as they look official.
For most of us, we're just beer-can racing, and even semi-serious amateur races do not really care that much what number you put down, as long as it's unique to their race. Most people put their hull numbers up there as it's sometimes meaningful and probably pretty unique in a field of thirty boats. There are very few rules about such formalities in amateur sports.