According to the standard syslog protocol defined by IETF RFC 5424, severity levels are used to categorize the importance of log messages. The two highest severity levels are Emergency (level 0) and Alert (level 1). 'Emergency' indicates the system is unusable, and 'Alert' signifies that immediate action is required. Both of these levels are reserved for the most critical system or network issues that demand immediate attention from administrators to prevent or address catastrophic failures.

C. Notice: This level (severity 5) is for normal but significant conditions that do not represent an error or a critical issue.

D. Debug: This is the lowest severity level (level 7) and is used to output detailed information for troubleshooting by developers or engineers, not for flagging critical operational problems.

1. IETF RFC 5424

The Syslog Protocol

Section 6.2.1. Severity: This document defines the standard severity levels.

0 Emergency: system is unusable

1 Alert: action must be taken immediately

5 Notice: normal but significant condition

7 Debug: debug-level messages

2. ArubaOS-CX 10.11 Fundamentals Guide

Chapter: Logging

Section: Logging severity levels: This official HPE Aruba guide lists the severity levels used in their network operating systems

which directly align with the RFC standard. It describes 'emergency' and 'alert' as the levels for the most severe events.