A minimal number of keywords has been defined as part of the FITS standard, but additional ones can be (and have been) added by various groups to support their own needs. Thus, several ``flavors'' of FITS coexist. A detailed description of the CLASS FITS flavors is given in chapter . Unknown keywords are normally ignored, but CLASS supports FITS keyword redefinition. If you receive a file with scan number coded as NUMBER (instead of SCAN-NUM), all you need to do is to define a SIC symbol named NUMBER with translation SCAN-NUM. This is done by typing SICSYMBOL NUMBER SCAN-NUM.