When using New for the first time it creates a copy of the <Master Design> and it becomes the 'default', after that point if used it will create a copy of the currently selected variant but for just component data such as Fitted/Not Fitted and Shared Identifiers only.
If Copy is used it copies everything including Variant Name, Description, Attributes and Text as well as component data. This is the one to use after creating the initial 'default' one using New.