Change in charge transfer efficiency as reflected by the gradual decrease of the uncorrected pulse height value for events from the onboard Al-Kα calibration sources. Major events: tuning of the CCD settings, cooling of the detector and switch to the single node read-out for RGS2, cause sudden shifts in CTI. Occasional outliers are caused by peculiar observations like e.g. bright sources, observations with high background and very short observations.