A totally uncustomised style that you have when you first install Xenforo is the default style.
So if you create some custom styles they are not the default style (although they may be based on the original default style.
When you tick a style in the list, it becomes the "default" style that people see unless they choose another from the style chooser.
However this default style doesn't have to be the default style you get when you first install xenforo.
If you tick the custom style, and then hide it from users by sliding the button, you get a message:
It is not possible to prevent users from selecting the default style. (even though it was not the default style, it was a custom style)
Still not confusing ??? (unless of course you already understand the two meanings of "default"
