Originally Posted by Infinite1K

The random assignment of N numbers is more of a Continental model. United had the NXXXUA format, with (roughly) each hundred block assigned to an aircraft type. N7XXUA was for 777, N4XXUA A320, N8XXUA for A319, etc.

This approach also had the added advantage so that if in the flight status you saw say a 777 as ship 2786, you would know it was N786UA, which would make it easy to do flight tracking based on tail numbers.

But with the Continental model, if a flight operated with a 777, and the ship was listed as say 3001, you couldn’t as easily figure out what the tail number was.