...
When we need N simulated events for a certain time period, we select N zero-bias events recorded during this time and look up the detector conditions at those N discrete times. These conditions could be the same for some period; they could also be different for every event. We then generate and simulate signal events according to these conditions, whatever those conditions might be.
After the signal events have been simulated, we add in the hits recorded in the corresponding zero-bias event to give the complete signal plus background sample. This step is referred to as overlay.
...