Extracted from: http://httpd.apache.org/docs/2.2/mod/mod_log_config.html#logformat

Format String Description
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%% The percent sign
%a Remote IP-address
%A Local IP-address
%B Size of response in bytes, excluding HTTP headers.
%b Size of response in bytes, excluding HTTP headers. In CLF format, i.e. a '-' rather than a 0 when no bytes are sent.
%{Foobar}C The contents of cookie Foobar in the request sent to the server. Only version 0 cookies are fully supported.
%D The time taken to serve the request, in microseconds.
%{FOOBAR}e The contents of the environment variable FOOBAR
%f Filename
%h Remote host
%H The request protocol
%{Foobar}i The contents of Foobar: header line(s) in the request sent to the server.
Changes made by other modules (e.g. mod_headers) affect this.
%k Number of keepalive requests handled on this connection.
Interesting if KeepAlive is being used, so that, for example,
a '1' means the first keepalive request after the initial one,
a '2' the second, etc...; otherwise this is always 0 (indicating the initial request).
Available in versions 2.2.11 and later.
%l Remote logname (from identd, if supplied).
This will return a dash unless mod_ident is present and IdentityCheck is set On.
%m The request method
%{Foobar}n The contents of note Foobar from another module.
%{Foobar}o The contents of Foobar: header line(s) in the reply.
%p The canonical port of the server serving the request
%{format}p The canonical port of the server serving the request or the server's actual port or the client's actual port.
Valid formats are canonical, local, or remote.
%P The process ID of the child that serviced the request.
%{format}P The process ID or thread id of the child that serviced the request.
Valid formats are pid, tid, and hextid. hextid requires APR 1.2.0 or higher.
%q The query string (prepended with a ? if a query string exists, otherwise an empty string)
%r First line of request
%R The handler generating the response (if any).
%s Status. For requests that got internally redirected,
this is the status of the *original* request --- %>s for the last.
%t Time the request was received (standard english format)
%{format}t The time, in the form given by format, which should be in strftime(3) format.
(potentially localized)
%T The time taken to serve the request, in seconds.
%u Remote user (from auth; may be bogus if return status (%s) is 401)
%U The URL path requested, not including any query string.
%v The canonical ServerName of the server serving the request.
%V The server name according to the UseCanonicalName setting.
%X Connection status when response is completed:
X = connection aborted before the response completed.
+ = connection may be kept alive after the response is sent.
- = connection will be closed after the response is sent.
(This directive was %c in late versions of Apache 1.3,
but this conflicted with the historical ssl %{var}c syntax.)
%I Bytes received, including request and headers, cannot be zero.
You need to enable mod_logio to use this.
%O Bytes sent, including headers, cannot be zero. You need to enable mod_logio to use this.