There is a new option under the Roles and Permissions settings for 2015.2, "Mobile Self Service UI" - what does this do?
There are two Mobile interface types available now (as of 2015.2). The 'old' mobile app and a new Web-based Mobile Interface for Android and IOS.
Since there is no sure way to know if a user is connecting through a mobile 'browser' that selection 'Mobile Self Service UI' allows you to make it the default UI for given roles (when selected). When users select that role during the Login process the Web-Based Mobile UI is presented for use on their Tablet or other mobile device.
The Mobile App itself does know when its being called due to it being an App so it does not need to be pre-set.
In order to give your Self-Service users (or any other Role) more flexibility - creating two similar roles where one uses the old (standard) Interface and the other uses the new Mobile UI is a very good option. That way the users can choose what interface they wish to use based on the device they are connecting through at the time.