Each message that is sent must have a source port. Normally this is automatically generated by the sending device. SyslogGen allows you to specify the source port if you want. Normally you would leave this option set to 'Random port > 1023'. SyslogGen will then allocate the source port for you each time.
Although in the Syslog RFC it is recommended that a source port should be the same as the destination port, it is not always possible to do this with Winsock (Windows Sockets). As an alternative, the RFC recommends that the source port be a consistent value for each message.
SyslogGen won't let you set the Source port the same as the Destination port because of the Winsock limitations.