The BACKSCAL keyword (commonly used in fitting packages for background scaling) in the single-ObsId source spectrum is computed by estimating the integral of the exposure map (supplied by the observer) over the extraction region. This deviation from the common practice of using the simple geometric area of the extraction region for BACKSCAL was chosen because it is felt this method provides more accurate background scaling in cases where the exposure varies significantly over the extraction region. Such exposure variation would come from CCD edges or masked regions dithering into the extraction region.
The EXPOSURE, ONTIME, and LIVETIME keywords in the source spectrum are set equal to the EXPOSURn, ONTIMEn, and LIVTIMEn keywords, where CCD n contains the most extracted events. In some observations where the CCDs have significantly different time filters applied, this leads to more accurate absolute flux values than would be produced using the default EXPOSURE value which corresponds to the aim-point CCD. This potential inaccuracy is described on the CXC web site.