In general any data include the time-variable background. If the background during a certain and limited period in the observation is too high compared to the averaged ratio of the intensity of the source to background, then those period with high background had better be excluded from the data. As the background level sometimes flares up during X-ray observations, we call it background (BKG) flare-filtering.
Now it is not a trivial question what is the optimum level of the BKG-flare-filtering.
