The combined log format is an extension to the Common Log Format. It adds informations about the user agent and referer. It is also known as the extended common log format. It was first implemented in the NSCA httpd web server but is now supported in many web servers. Apache™ can be configured to use this log format.
Two fields are added at the end of the common log lines:
"referer" "useragent"
The content of the Referer header of the request. This usually reflects the page the user visited before this request.
The content of the User-Agent header of the request. This usually reflects the browser that the user is using.