$body_bytes_sent, the number of bytes, transmitted to client minus the response headers. This variable is compatible with the %B parameter of Apache's mod_log_config (this was called $apache_bytes_sent, before version 0.3.10)
$bytes_sent, the number of bytes transmitted to client
$connection, the number of connections
$msec, the current time at the moment of writing the log entry (microsecond accuracy)
$pipe, "p" if request was pipelined
$request_length, the length of the body of the request
$request_time, the time it took nginx to work on the request, in milliseconds, (in seconds for versions older than 0.5.19)