Reference, here: LOGMODULE
$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 connection
$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 seconds with millisecond precision (just seconds for versions older than 0.5.19)
$status, status of answer
$time_iso8601, time in ISO 8601 format, e. g. 2011-03-21T18:52:25+03:00 (added in 0.9.6)
$time_local, local time into common log format.