Also, companies don't necessarily start at "1" - they may start at 100 or 1000 or something else (if only to avoid "leading zeros", like 001). Some will design numbering systems with the some digits representing size (eg tonnage or length), the year of purchase, the type etc. Capt Ted picked the best example - Kirby alone have nearly 1200 barges on the US register that are numbered just like that.