Tracks no longer has to be numbered from 1 to [number of tracks]. This is especially useful when adding audio files and you do not have all the tracks from the album. If you would the like program to automatically number the tracks from 1 to [number of tracks] when adding audio files (like in previous versions), check the "Reset track index" options in the Audio File Options window.

You can manually set the track index from the Set Track Index window: