This option in Work Environment / More Options will allow you to open a new instance if you're selecting from recent list or dragging a file onto the Archicad icon, but if you have saved your current file I don't think this will override the default action of opening in current instance - you will still have to check the "Launch a new instance" button. I try to make sure this is the first thing I do if I want a new instance, before selecting the file.