The spectrum is usually accumulated by the user over a specific region or regions in space. In order for the ARF generated to be appropriate for that region or regions, arfgen extracts spatial information from the Data Sub-Space (DSS) stored in the spectral dataset (see the dsslib documentation for more information regarding the DSS). This task uses the regions to calculate the spatially dependent elements of the ARF.
The DSS information is only written to the spectrum by evselect (or xmmselect) during its creation, if the evselect parameter writedss is set to true, and if the user has specified a region expression via the evselect expression parameter that is anything other than true or null. If there is no DSS information present an ARF will not be calculated.
Note that arfgen only handles regions, for imaging mode data, defined in POS (X/Y) or DET (DETX/DETY) space; so specifying a region component in terms of any other axis or axes, eg RAWX/RAWY, will cause the task to fail. Timing and Burst mode regions, on the contrary may be specified in terms of RAW coordinates.