It’s based on downloads. It is easy to track those.
“We are introducing a Unity Runtime Fee that is based upon each time a qualifying game is downloaded by an end user. We chose this because each time a game is downloaded, the Unity Runtime is also installed. Also we believe that an initial install-based fee allows creators to keep the ongoing financial gains from player engagement, unlike a revenue share.” blog.unity.com/…/plan-pricing-and-packaging-updat…