The first time you open Webmail, you may be asked to choose an interface. You may also notice that you can set your choice as default ( the “open my inbox when I log in” checkbox) so that you do not have to choose again the next time you log in.
Choosing a default Webmail interface can be confusing, especially if you do not really know the differences between the different options.
Please be rest assured that these differences are fairly marginal. Apart from aesthetics, there are no useful functional differences between the available options.