20-25
Indicate system problems and are fatal errors, which means that the Database Engine task that is executing a statement or batch is no longer running. The task records information about what occurred and then terminates. In most cases, the application connection to the instance of the Database Engine may also terminate. If this happens, depending on the problem, the application might not be able to reconnect.

Error messages in this range can affect all of the processes accessing data in the same database and may indicate that a database or object is damaged. Error messages with a severity level from 19 through 25 are written to the error log.

Here you can find another controversial link which says that severity level of messages between 1 and 25.

Carlo Romagnano (4/16/2009)

---

Correct answer is 1-25:

exec sp_helptext sp_addmessage

in the output search for @severity and you'll find this piece of code that is the law.

-- Valid severity range for user defined messges is 1 to 25.
if @severity not between 1 and 25
begin
raiserror(15041,-1,-1)
return (1)
end

Christian Buettner (4/16/2009)

---

No, 0-25 is correct. The fact that you cannot use 0 for sysmessages does not mean the severity level does not exist.