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Usage guidance, also known as a tag wiki excerpt, is a short blurb that describes when and why a tag should be used on this site specifically.
Standardized by the RFC 5424, it is a de facto standard to manage system logs on network assets and Unix-like operating systems. It is a client/server protocol used to centralize log messages, but many times configured to deliver local only logs.
It have facility codes that define what type of sofware created a log entry and severity levels to better separate messages to be interpreted by a sysadmin, or integrate with a monitoring tool.