Well, I'll be honest -- I have very limited experience with this, but I'll provide what is basically an educated guess based on what I have read here in the past and on a little bit of research that I just did now when I read your question.
For context, I'll start by going the other way -- saving an existing User Song, meaning a song you recorded using the multi-track sequencer, as a Midi file. As far as I know, for the 16 tracks of the Midi file, it allocates the first 8 to the melody tracks, and the second 8 to the accompaniment. And to go deeper, those first 8 tracks are based on the main and dual voices of the 5 melody tracks. Of course, that would give you 10 tracks (5 sets of main and 5 sets of dual), not 8 -- so I believe 2 of the dual voice tracks are not saved. Which ones? I'm not sure -- but if I was to search for the missing tracks, I'd start with the dual voices of tracks 4 and 5. I'm also not sure what order these tracks are mapped to Midi channels. One possibility is that they are done in pairs -- where the keyboard's track 1 main and dual voices are send to Midi channels 1 and 2, and the the keyboard's track 2 main and dual are sent to Midi channels 3 and 4, and so on. But maybe all the main voices grouped together and sent to channels 1 through 5, and then 3 of the dual voices are sent to channels 6 through 8. Again, I'm just guessing here.
The accompaniment, as far as I know, is then sent to the remaining 8 Midi channels, or channels 9-16. Now, the keyboard shows 6 parts for accompaniment on the track buttons -- track parts for bass, pads, drums, and so on. I believe that some of these keyboard tracks are split into two channels, so that the 6 accompaniment tracks from the keyboard are sent to 8 Midi channels.
So, with that context, my guess would be that Midi songs that you load would be mapped in a similar way. On my PSR-E433, there are 100 pre-recorded songs built in, and I experimented with some of them to see what happened when I turned off and on the different tracks using the track buttons. One of the surprising things is that the accompaniment track can be quite complex. It does not have to be a simple repeating pattern or style. I guess, when creating a Midi song on a more advanced external source, you can set those accompaniment tracks to be essentially up to 8 additional counterpoint melodies -- as the accompaniment track is likely mapped to 8 Midi channels -- channels 9-16 -- as far as I know.
So, this is my guess. I could be way off -- like I said, I have never really messed around with Midi too much. So if anyone else here has any input on this or corrections to what I stated, please post what you know.
But otherwise, you can experiment by taking a Midi file -- one where you know what instruments are on what channels -- and then load it into the keyboard and see what happens. Turn on and off the various track buttons on the keyboard while the song is playing to see what track buttons affect what instruments, and that can help you determine which Midi channels are mapped to which keyboard tracks. One other possibility is that Midi songs loaded in the keyboard can only be played in full, without the ability to mute the individual tracks. Like I said -- the best way to see what is possible is to load a Midi file and see what happens when you turn the track buttons on and off.
ADDT'L INFO... Keep in mind that, as far as I know, there is no way to edit imported Midi songs on the keyboard itself. They don't get loaded into the User Song memory (where you make your own recordings), but instead just become additional prerecorded songs you can play, just like the included demo songs, as far as I know.
