A user account allows one to authenticate to system services. It also generally provides one with the opportunity to be authorized to access them. However, authentication does not automatically imply authorization. Once the user has logged on, the operating system will often use an identifier such as an integer to refer to them, rather than their username. On Unix systems this is called the user identifier or user id. Computer systems are divided into two groups based on what kind of users they have: * single-user systems do not have a concept of several user accounts * multi-user systems have such a concept, and require users to identify themselves before using the system.