When an X-ray photon hits an SIS CCD, it generally activates more than one pixel. The pattern of activated pixels can be used to help distinguish spurious events such as cosmic ray hits.
The spacecraft has two modes in which it can send information to the ground. In FAINT mode separate data from each pixel is sent to the ground. This has the advantage of preserving the greatest amount of information, but the disadvantages that a high telemetry rate is required and that FAINT mode data cannot be interpreted by current analysis software. In BRIGHT mode, the nine pixels around each event are grouped together and on-board software interprets which events are valid. Some information is lost, but a lower telemetry rate is required and the data are directly interpretable by current analysis software.