evselect will optionally update exposure information for event lists and and product files (no exposure information is written to histograms). This can be done only for finalized XMM event lists, and if the event list does not have the right structure, a warning will be issued. By default exposure updating is turned on. It can be turned off by setting the boolean parameter updateexposure to false. Note that data subspace writing must be turned on in order for exposure correction to work correctly (writedss set to true).
For EPIC event lists, evselect uses information in the data subspace and in the EXPOSUnn extensions of the event lists to update the ONTIME, ONTIMEnn, LIVETIME, and LIVETInn keywords. Here nn refers to the relevant CCD number. For spectrum and rate files, evselect determines the EXPOSURE keyword by doing an average of the live times of the CCDs in the selection region, weighted by the number of selected events in each CCD. For images, the the EXPOSURE keyword is set to the maximum value of all the available ONTIMEnn values.
As the EXPOSUnn extensions for an event list can be quite large, evselect can filter these extensions, using the same timing information as is applied to events from the corresponding CCD. This option is controlled by the parameter filterexposure, which is set to true by default.
Exposure correction cannot be performed for the RGS within evselect. Instead, evselect simply propagates exposure information for RGS to any spectral files produced. A warning is issued if any TIME filtering is done for RGS event lists, as this would invalidate the RGS exposure information.