Steve Stedman handles an error:
SQL Server uses error severity levels to indicate the nature and seriousness of an error. These severity levels range from 0 to 25, with each level indicating a different type of issue, from informational messages to catastrophic failures. In this post, we’ll focus on severity levels 19 through 25, which are typically the most critical errors that can occur in SQL Server.
Fortunately, given how devastating they are, severity level 21 and higher are extremely rare in my experience.