XMM data calibration is based on the use of the ``Current Calibration File'' (CCF). A CCF is the collection of all calibration files for the calibration of data from all XMM science instruments that are valid at a particular time. Each calibration file, which is a constituent of a CCF, has an issue number and a start-of-validity date by which it can be identified uniquely. However, a CCF does not have a version number or validity date, because it is a mere collection of calibration files carrying individual identifiers that are valid at a given time. The most up-to-date version of a calibration file is always the one with the highest issue number. Calibration files account for properties of either the XMM spacecraft, the X-ray telescopes or one or more of the payload instruments.
With each ODF, observers receive a corresponding CCF.
A detailed description of the CCF concept, including a list of the calibration files constituting a CCF, is currently in preparation as part of the XMM Data Files Handbook (in preparation).