Last year, Apple did something out of character: it introduced the usual update to the iPhone, the iPhone 8, while simultaneously introducing the iPhone X with a flurry of hype. The iPhone X was a total rethinking of the iPhone, with a new gesture system, Face ID unlocking, and an edge-to-edge screen that prompted a wave of app redesigns. The iPhone X was meant to usher in the future of phones...
Categories, unlike tags, can have a hierarchy. You might have a Jazz category, and under that have children categories for Bebop and Big Band. Totally optional.