The SAS tasks processes input and output data in the form of datasets. These are usually saved on disk as FITS files.
FITS files have a primary header, a primary image array and optional extensions, each with a header. In the case of the XMM-Newton FITS files, the primary header contains information about the mission, the instrument, the observation mode, the investigator, the target coordinates, the observation date and initial processing information. In all FITS headers, the information is in the form of keywords with assigned values.
It is important to note that the naming convention of XMM-Newton files is slightly different from that historically used for FITS files. Table 2 provides a handy conversion table between the two nomenclatures.