When you specify a severity level, messages of that severity level and greater are sent to the target. If you want only those messages of the specified severity to be sent to the target, use the keyword only. For example, specifying severity warning will send warning, error, and critical messages to the target, but specifying severity warning only sends only warning messages.
When you specify a severity level as you associate a filter with a target, you further restrict the messages reaching that target. The filter may allow only certain categories of messages to pass. Only the messages that pass the filter and then pass the specified severity level reach the target.
Finally, you can specify the severity levels of messages that reach the target by associating a filter with a target. The filter can specify exactly which message it will pass. Constructing a filter is described in Filtering By Components and Conditions.