I have noticed that the two most common sail numbers are hull number or a 5 digit number. How is this 5 digit number derived?

We're ordering new sails and of course this is the best time to confirm the sail number.

My TSP is TSP300870679 (J30 - hull 87 - June 1979).
Hull # 87
US Coast Gaurd Hull Identification Number 614825
Rampage sail number 26245 http://noyc.org/images/uk1.jpg

What is recommended?