<< So I think what you are explaining Charlie is that changing style via Bar Settings (F5) does not carry the styles part setting with it. In other words, what was initially a or b won't be changed to the a or b of the new style. >>
Yes it can contain the various style parts. A MultiStyle is notated by the + sign. A MultiStyle + will be composed of several single/normal styles that are compiled into the MultiStyle.
So -
MDWEST + Multistyle contains the following styles
MDWEST1 will have -a- and -b- substyles
MDWEST2 will have -a- and -b- substyles
MDWEST3 will have -a- and -b- substyles
Using F5 Bar Settings or the Part Markers Define Style option the user will select the different a or b substyles from any of the numbered MultiStyle's Sub Styles it's composed of. The a and b parts of each numbered substyle can't be mixed with other numbered substyles. Each numbered substyle remains part of that particular numbered style.

Numbered styles can be used as stand alone styles as well as being part of the MDWEST+ style. They can also be included into any custom style a user compiles.

Each of the numbered substyles can have different instruments replace instruments of other numbered styles and styles can replace instruments with RT1152:Silence to mute that instrument by changing Part Markers in the main SGU. For instance, MDWEST1 has all the instruments of MDWEST+. MDWEST2 replaces the bass and Pedal steel with RT1152 and MDWEST3 replaces RT1152 and returns the Bass and also replaces a background pedal steel with a soloist pedal steel. These arrangements can be placed in any order and at any time in the Main SGU by changing Part Markers.

Once in the main SGU song file, then they can be mixed because they are defined by a new substyle letter. The Part Markers in the main SGU file can be configured to any combination the user selects. In the main SGU file, adding MDWEST1, 2 or 3 the -a- and -b- substyles will have new letters assigned in the Main SGU program. You can see this in one of the Screenshots I attached where Trepid a/b substyles were re-lettered to -g- and -h- in the main SGU song Chord Chart and the Rainin MultiStyle that uses letters a through f.

<< Does this then mean that you can't have both a and/or b etc. parts from each style in the same SGU file? >>
Not at all. you can have both a and b parts from each style as explained above in the same SGU file.

<< If so, then it looks like Muso's suggestion to just use the DAW plugin with two (or more) style versions of the same song in the DAW and arrange from there is the way to go. >>
This method works fine with the exception it's more time consuming, more complex and one loses all of the features like regeneration and rearranging the Part Markers by BIAB once files are exported out of the program into a DAW.

Attached Files (Click to download or enlarge) (Only available when you are logged in)
MDWEST+ MultiStyle.jpg (181.27 KB, 129 downloads)
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.