I don't think that the internal workings of Natural Arrangement are described.

The manual states:
"Natural Arrangements (re-interpret complex chords): If you give a pro musician a complicated chord progression
with fancy extensions like C7b9b13 or Gm11, the musician may reinterpret these rather than playing them exactly as written. This can achieve a much better sounding arrangement because the musician has freedom to choose from similar chord extensions. You can get Band-in-a-Box to do the same thing with this option for all tracks in all songs."


The intention is to provide a variation of how complex chords are presented.

What you are hearing at bar 19 is presumably the artistic license of the performer (a simple embellishment), as your song has no complex chords.


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