Reading about the user tracks feature, I get the idea of playing chord tones over chords and have learned it's helpful to record UT's over progressions I will be using.
My first question is this: Should user tracks be very basic rhythmically? Does BIAB change the rhythm to match the style or does it need a swing 1/8th note UT to sound right for that style?
Second question: Will BIAB add appropriate non recorded non chord tones?

The audio in a usertrack is whatever you record to the audio file, no more or no less. If the song you are making needs eighth notes you have an issue because the usertrack SGU file documents chord positions within the audio, not notes. At the same time remember if you document a G7 chord at bar 4 of the usertrack SGU file and record a G7 eighth note guitar fill at bar 4 of the audio file then the G7 eighth note guitar fill will be used each time you use the usertrack in a song.

The SGU file tells BIAB or RealBand tempo and where chord audio information is located within the audio file. Besides those two deductions, the answer gets much murkier.

Style selection used during the creation of the usertrack SGU file doesn't seem to matter. If you are using BIAB or RealBand to record your audio track they will create backing tracks with style selected instruments just as they normally would during recording. But, remember you don't have to use BIAB or RealBand to record your audio track, you can use any daw you want to use.

Besides the chord location and tempo information from the SGU file, BIAB and RealBand seems to get the rest of the information needed to play the usertrack from within the options.txt file contained in the usertrack folder.

Just for fun I used the Clicktrack style and a blank style to create SGU files and substituted each in a usertrack folder. The usertrack continued to work fine in both cases. That pretty much put to rest my questions about interaction between a style used to create a usertrack SGU file and the style used to create a song.