Here are all the % sequences allowed in the configurable log format in Apache.
%b = bytes sent, excluding HTTP headers
%f = filename
%h = remote host
%{Header}i = The contents of Header: header line(s) in the request sent from the client
%l = remote username (from identd, if supplied)
%{Note}n = The contents of note "Note" from another module
%{Header}o = The contents of Header: header line(s) in the reply
%p = the port the request was served to
%P = the process ID of the child that serviced the request
%r = first line of request
%s = response status. For requests that got internally redirected, this is status of the original request: use %>s for the returned status
%t = time, in common log format time format
%{format}t = The time, in the form given by format, which should be in strftime format
%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
%v = the name of the server (i.e. the virtual host)