The team should feel free to correct me, but I have the feeling that they left the UI in place for features yet to be implemented — a bit like some of the techniques that are in the dialog but that you can’t map yet. To my experience, you can only have one master page of each kind: First, Default and Custom. Basing a master page in one of these templates replaces the original. My guess is that we will be able to have several instances of each kind, but not right now. What you can do is create a Custom master page, provided you’re not using that class already.