Hi Andreas,
calibration should be performed for individual users and for different reading or visual aids (glasses or contacts). It is not necessary to calibrate for each session. But note that the calibration is lost when the eye tracker is power cycled or disconnected. (Unless you save it, of course, which is what the EyeX Engine does.)
Also note that if you’re using the EyeX SDK then you should let EyeX take care of all user setup. But we’re considering opening up a configuration API that would let you, for example, switch between user calibration profiles, as a future extension.