Suppose you make a track with four channels. You use NNAs in the instruments. For example, you use Note Off. Now, suppose that, at some point, you make two four-note chords, one immediately after the other. As the instrument you're using has Note Off, each note playing in a channel doesn't get cut when the new note arrives. It keeps sounding for a while. So, in reality, although both notes are in the same channel, you hear two sounds at the same time for a while. You have two audio channels for each pattern channel. These audio channels are called virtual channels. In this case, your track has four channels, but in the point were the two chords are, it has eight virtual channels. Whilst the number of pattern channels is fixes, the number of virtual channels varies, according to what needs to be played. The maximum number of virtual channels that can be handled depends on the power of your CPU. If the number of virtual channels the track needs exceeds the maximum your CPU is capable of handling, the ones with least volume are simply discarded. In our example, suppose your CPU can manage only seven chanels. Then, one of the notes, the one with the least volume, will be cut.
