I've pretty much ignored this button since I set up worthwhile views within my models.
However, this user defined option seems to say how new views will display by default within the current session.
You can set this view creation dialog to "user defined" and it becomes a top view; then you can rotate about x and y according to the angles below the user defined dialog box.
Again, these are default views. I suggest never setting the default to Isometric and using this as a final view orientation. Anytime that the isometric default is switched back to trimetric, all your views named "default" will also change from isometric to trimetric.