The local channel (l) is used to deliver messages to addresses on the local host.
When using a mail user agent on the local system to send mail (to anywhere), the sendmail utility (/opt/SUNWmail/imta/bin/sendmail) is invoked as the replacement for sendmail to queue the messages to the appropriate queues, and then the channel programs for those queues will process the messages.
The IMTA configuration utility always generates a local channel.