Which severity level is used to indicate that a system is unusable?

The severity level that indicates a system is unusable is "Emergency." This classification reflects the highest level of urgency and severity, typically signifying that the system is down or in a state that prevents any normal operation. In practice, when an emergency occurs, immediate action is required to restore functionality, as operations are severely impacted and may halt altogether.

In contrast, the other severity levels serve different purposes. A warning indicates potential issues that might escalate but do not currently impede system performance. Critical severity is serious and indicates significant problems that could cause major issues, yet the system may still be operational to some extent. The info level simply represents informational messages, which do not imply that any immediate action or concern is necessary. Understanding these distinctions is essential for effective IT service management and incident response.