Some playback styles have three instrument tracks, others have four. Where there are four, the additional track provides contrasting rhythms, voicings from the other, so they can be mixed in different ways to provide different backing sounds according to each instrument's selection (e.g. organ + guitar) or as an alternative, mute one of them. This means the same style can sound different from another song using the same playback style, because of different instruments are used and/or mixed (volumes) differently.
Both are provided in the midi file and you can assign whatever instruments you like to them, edit, set volumes, pan, reverb, other effects etc.

BTW, the drum track is our internal track specifically related to our drum and percussion note mappings within iReal Pro. (It does not adhere to the General midi percussion map). Generally users like to find their own drum loops if using a DAW.