Thank you both for your helpful replies.

Child processes seem to have a zeroed pr_CLI and, from what I've observed, that typically results in the assumption that the program must then have been started from Ambient - after which WaitPort() and GetMsg() are typically called by the program on its own pr_MsgPort in anticipation of an initialisation message.

If a parent process creates such a child process, do I assume that the parent has to send this initialisation message to the child?