Regions
Regions are two dimensional filters that can be used to include or exclude data from a given file. They may be applied to any column in a file (time, spatial, etc.), using the DM command-line syntax. Regions can be read and written by the ds9 imaging tool and read by CIAO tools (e.g. dmextract, to extract photons just from the sources).
Regions are made up of a number of shapes that are listed in the dmregions ahelp page. The shapes can either be inclusive or exclusive. The different shapes can then be combined using either a boolean AND or boolean OR operation which leads to a degree of flexibility to design arbitrary regions.
For more information on creating and using regions, see the following threads:
