This is the radical addition. The 8th consciousness still contains seeds of delusion. When those seeds are completely purified—when the storehouse is emptied of all defilements—what remains? That’s the 9th consciousness: Amala-vijnana , or "undefiled consciousness."
Beyond the Mind: Understanding the Nine Consciousnesses in Yogacara Buddhism nine consciousnesses
This is your everyday thinking mind. It integrates the five senses, conceptualizes, judges, plans, and remembers. It’s the "chatter" you identify as "you." This is the radical addition