Unity's licensing model operates on a revenue or funding cap basis. The free version, Unity Personal, can be used by individuals, hobbyists, and small businesses whose total gross revenues (from all sources, not just Unity-based) or total funds raised are less than $100,000 in the last 12 months. If your revenues or funding exceed this amount, you are required to purchase a subscription to Unity Plus or Unity Pro. This is what's commonly referred to as the "100k rule."