[U-238]

If I am correct,the FAA assigns the tail number and there are different categories. I may be wrong.

[DaveArk]

Tail numbers are assigned according to the year the contract is signed for the order. My first F15 was tail number 75033. The contract order was signed in 1975. It didn't come off the line until 1977. it gives the company a specific airframe to order from all the suppliers the various parts.