I understand that not all Tracks have a Simple Arrangement option so selecting it may not always give the desired result.
RealTracks will have a letter "s" in the "Simpler Available" column in the RealTracks Picker if the RealTracks instrument has simpler options available.

I'm not sure, but I think the global / song level option chooses all RealTracks that have the Simple Option available, whereas at Track Level the user can individually specify.

The Help states: "If Simpler is checked, the selected RealTracks will play a simpler arrangement (less busy, less embellished). "

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