Incoming photons which hit more than one SPI detector are called multiple events. A set of ``pseudo-detectors'' are defined to incorporate double and triple events into the analysis. Simulations showed that events hitting two detectors originate from a relatively small area (smaller than that of a detector) located near the border of the two detectors. The same is true for triple events, they occur very near the common corner of the three involved detectors. Hence the idea of considering these small areas located between detectors as ``pseudo detectors''. The response of these ``pseudo detectors'' is computed from GEANT simulations, and they are included in the analysis as if they were real detectors. Events hitting more than 3 detectors are increasingly dominated by background events. They are not normally used in scientific analysis.
In general for analysis up to a few hundreds of keV, the inclusion of the double and triple events does not significantly improve the results. In other words, for point sources, most analysis can be restricted to ``real detectors'', i.e, a detector list 0-18. It is probably always a good idea to start an analysis first using only real detectors anyway. For high-energy analysis, double (pseudo-detectors 19 - 60) and triple (pseudo-detectors 61 - 84) can be added later into the analysis, using an extended detector list. Note however that the current RMF response available for XSPEC spectral response is valid for analysis with real detectors (0-18) only.