If I understand your first post correctly when you say, "... create a song that completely combines multiple styles, all the instruments from every part, from beginning to end, even if that would be discordant or redundant."

MultiStyles will do exactly that. Substyles are derived from a master style, using Master style for want of a better word. I don't recall what PG Music calls them at the moment. Any existing style can be modified into a substyle or a substyle can be another existing style or a style that's custom user created.
A master style will be identified by a + following the style name.
The modified substyles will be identified by a number following the style name

From the Master MultiStyle, the substyles will be activated by the different colored Part Markers. There can be up to 24 different Part Markers in any single song.

I've attached a screen shot of a PG Music created MultiStyle. There are nearly 1,000 PG Music created MultiStyles in the StylePicker on my version of BIAB. However, any style can be modified into a MultiStyle.

There are two types of MultiStyles, song specific MultiStyle and a Style MultiStyle.

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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.