EDTracker models 3 degrees of freedom (3DOF) - pitch, yaw and roll. So your head twisting and looking around is what is monitored and reported.
It does not report lateral or vertical movements of your head - so any sideways movement of your head, or moving it towards (or away) from the screen. This is what is commonly referred to as 6DOF.
Note: do not confuse true "degrees of freedom" with the marketing hyperbole touted by the sensor manufacturers. For example, the Invensense MPU-6050 is touted as "6DOF"; the 9150 as "9DOF". This only represents the number of sensor channels that are provided - in the 6050 sense, this is 3 axes on the gyroscope and 3 axes on the accelerometer. The 9150 adds another 3 axes for the magnetometer. Other sensors add a pressure sensor and claim "10DOF"! This has nothing to do with true "degrees of freedom" and motion/movement capturing, and these enhanced sensors still cannot deliver truly accurate 6DOF to the user experience.