Well, you read the title. There is not much else to it. This makes Final Fantasy XV the fastest selling title in Final Fantasy History. ( Should note that it is ‘Shipments’, which are sales for the company)
This means the game in its first day has sold a little under half of what Final Fantasy VII has. VII is still the highest selling in the franchise. But those sales are over a long period of time. Maybe XV will come to pass it eventually. Time will tell.