User Identification Number is used for identification of users.
Identification by nickname is not possible, because many users can have the same one.
There are usually nine digits in ICQ, sometimes divided to three groups of three (123-456-789). Some UINs have less digits, no number starts with zero.
User can't choose or change his UIN, he is granted an unused one (imho maybe the lowest of them). Because one billion's range is limiting, UINs that are not longer used are granted to new users.