Unity operates on a subscription model. The free version of Unity can be used by individuals and small businesses that make less than $100,000 per year. If your company's revenue or funding exceeds that amount, you'll need to purchase a Unity Pro license. There is no royalty fee based on the revenue of a game or application developed with Unity. However, if your game generates substantial revenue, you may need to upgrade your subscription. Always make sure to review Unity's current licensing agreements for the most accurate information.