Users need to identify themselves for accounting, security, logging, and resource management. Usually a person must also enter a password in order to access a service. Once the user has logged on the operating system will often use a (short) user identifier, e.g. an integer, to refer to them rather than their user name.
User names can usually be any short string of alphanumeric characters. Common choices are first name, initials, or some combination of first name, last name, initials and an arbitrary number. User names are often assigned by system administrators according to some local policy, or they may be chosen by the users themselves.