Let me be clearer. The use of /var/log/xferlog is defined by your syslog.conf(5) file. The default for that is anything syslog is sent, by
any daemon, with selector field entries of "ftp.info" or "ftp.debug".
That daemon can send anything it wants in a character message, there is no standard.
Any text string at all, within the limits of a printf(3) string. See syslog(3) for the programming details.
That stated, ftpd(8) writes to a number of logs, because it uses various selector fields. The format of its ftp.info and ftp.debug syslog messages may be found in its source code -- if you have the source code on your system it will be in /usr/src/libexec/ftpd -- the files of interest are ftpd.c for ftp.info messages, and ftpcmd.y, ftpd.c once more, and monitor.c for ftp.debug messages.
If you don't have the source code, and don't feel like installing it, you can dig through the CVS repository for src/libexec/ftpd on the web starting here:
http://www.openbsd.org/cgi-bin/cvsweb/src/libexec/ftpd/