<< If I can open a frozen multistyle SGU in Reaper then the Plugin can be made to do the same thing giving the user the same as the main Biab app in their DAW. >>

That may be true. I don't have Reaper to test. You'll need to test with the Plug-in. There's no waveform to see because there's no waveform generated.in the plug-in and the plug-in doesn't recognize any Part Markers other than the normal blue and green a) and b) sub-styles. I don't doubt that it can be done, but I haven't been successful using the Plug-in or now, RealBand. I don't see how the WAV instructions will read the additional Part Markers if the Plug-in and RealBand isn't reading them. Is that possible?

In your first image in Reaper, it also looks like it rendered the 2nd drum pattern when the trumpet part comes in. That's awesome.
Since the discussion was on the Plug-in I didn't test with RealBand. So now I have. I loaded the MultiStyle Demo and posted a screen shot. RealBand did not generate the Multi-Soloists track at all.
I opened the MultiStyle Demo in the Plug-in and posted a screen shot. The Plug-in did render the Strings Track that contains the multiple soloist instruments but there's no sound output. I attached a screen shot of the Plug-in's rendered Strings Track saved audio file.

I can see the rendered tracks correctly in BIAB and they render and play.

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BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.