MIDI files contain up to 16 channels of MIDI commands and are written in such a way to support playback of those songs on a MIDI-compatible instrument. OnSong supports MIDI files as backing tracks in OnSong which can be linked to chord charts and played. This allows synthesizer apps as well as virtual and MIDI-enabled instruments to playback the MIDI stream. You can also use this to configure more advanced MIDI commands to be sent for configuration when a song loads or when you trigger playback.
When you import audio files directly into OnSong, they are added to your OnSong audio media library where you can link them to chord charts as backing tracks.