Simulation corresponds to the process of writing data, except that no data is written. Simulation is used to check whether the data can be sent to the recorder quickly enough to avoid a buffer underrun. It can also be used to check whether the recorder can actually write to the CD. You should use simulation when you are not sure whether the system can send the data quickly enough. If you have already burnt a few CDs successfully, you can switch off simulation. If your recorder has a buffer underrun protection function, you do not need to use simulation.