In the Gui4Cli source this part reacts to a change in console output specification
(con:0/0/.. etc)
done with the Gui4Cli interpreter command
Set output

	case OUTPUT :
Printf("Set Output %s %s",param, val);
		if (DEFcpOPEN) Close (DEFcp);
		DEFcp = ZERO; //OS4 was NULL, Global variable
		DEFcpOPEN = 0;
		if ((DEFcp = Open (val, MODE_OLDFILE)))
		{	
			Printf("Set Output  %s opened",val);
			SelectOutput (DEFcp);
	 		DEFcpOPEN = 1; /// global variable too
			stccpy (Defconsolespec, val, 125); 
		}
		else Printf ("*ERR: Invalid Console spec:\n->%s", val); // we do see this
		break;

The Gui4Cli user doc doc only considers respecification of the console.
However ( i don't have a printer connected)
set OUTPUT PRT: works if the printer preference is set to using printtofile.device.

Only problem there i can't get rid of an initial requester asking to define the output file.

I tried to use
set output RAM:output
but got the erroor message above, i don't understand why