The EyeX Engine is a runtime that extrapolates and filters eye-gaze and eye position data from the eye tracker, and presents it i a nice way both as data streams but also as higher level interaction concepts through the EyeX Engine API. The API is designed for integrating eye-gaze interaction into applications and games. An example of a higher level interaction concept is the Activatable Behavior, where a client application would define a certain area (for example a button) as Activable and the EyeX Engine would send interaction events to inform the application when the user focuses the button for interaction but also when the user activates (or “clicks”) the button using eye-gaze in combination with pressing the designated EyeX Button on the keyboard.
I would say that the EyeX Engine doesn’t really record tracking data, but has a “short time memory” of data to be able to filter eye-gaze data and interpret what the user is trying to do. It only relays information about key presses and such implicitly, since EyeX events and state changes can be initiated by key presses and other external events.
There is no pupil size information available through the EyeX Engine API.