The behaviour of this form of the CustomLog
directive is almost identical to the standard
CustomLog directive. The difference is that the
'env=' clause controls whether a particular request
will be logged in the specified file or not. If the specified
environment variable is set for the request (or is not set, in the
case of a 'env=!name' clause), then the
request will be logged.
Environment variables can be set on a per-request basis using the mod_setenvif and/or mod_rewrite modules. For example, if you don't want to record requests for all GIF images on your server in a separate logfile but not your main log, you can use:
SetEnvIf Request_URI \.gif$ gif-image
CustomLog gif-requests.log common env=gif-image
CustomLog nongif-requests.log common env=!gif-image