That's what I said. "Optional parameter." That is, the old version continues to work without any channel(s) specified, and when channels are specified then it works differently.
The difference being that it will be multi-fired in a channel context; $chan is populated by each channel name, behaving the same as someone parting many channels at once. The same QUIT from the server would fire On Quit = $comchan($nick,0)-times.
