All Ribbon tabs in PowerPoint 2016 may have any number of buttons that represent commands but they are not scattered all over the tab area. In fact, they are all neatly arranged together in Groups. Each of these Groups has a name that describes what the commands within that Group do. For example, the Slide Show tab in the Ribbon has a group named Set Up, which contains all commands that help you set up your slideshow, such as changing show settings, hiding slides, rehearsing and recording slideshows, etc.
It is only sensible to imagine that you should use this Group concept while creating your own custom Ribbon tabs. In fact, PowerPoint will not let you add any command anywhere else other than within a custom Group.