The regions can be specified via the outunit parameter in either of two projection-plane coordinate systems, namely `detxy' and `xy'. (This parameter is not case-sensitive.) The first choice corresponds to columns DETX and DETY of the standard XMM EPIC event list, the second to columns X and Y. The DETXY coordinate system is fixed with respect to the EPIC instrument and is equivalent to the CAMCOORD2 system described in the cal task documentation, except that the units are 0.05 arcseconds instead of mm. Hence the DETXY projection plane is normal to the optic axis of the telescope. The XY system is fixed with respect to the sky (apart from any errors in the attitude correction); its units (= the size of its `pixels') happen also at present to be 0.05 arcseconds, but could in principle take any value since region makes no assumptions about them.
Note that in either case the task needs to know information about the EPIC instrument and the epoch of the observation. This information is obtained from keywords in the header of the supplied event list.
If the user specifies outunit=`xy' then the task also needs WCS keywords to define the connection between the source RAs and decs and their positions in the XY coordinate system. These keywords are obtained from the X and Y columns of the event list.
If outunit is given as `detxy' and if the supplied source list is a standard XMM EPIC product then in principle the event list is unnecessary. However it has been decided for simplicity to make it mandatory to supply both the event list and the source list to region.
Note that if the event list to be filtered by the regions is different from that used to supply the WCS and other keywords, it is important that at least the INSTRUME keyword in the headers and (if outunit=`xy') the WCS keywords in the X and Y columns of the two event lists are the same.