That's all set in the F5 UserTracks Development, if you can chord and key it out, record it, then go into the settings.
It helps if you have the file in RealBand or you DAW with a 2 bar offset for the count-in so the bar numbers line up, then you can play it while setting the F5 UserTracks settings in Biab.
So if a riff you want Biab to play starts on 9 and ends on 12 you can
Don’t start a new Riff on this bar 10-12,

If you want you can add them to
Only choose this bar if the key matches 9-12
and/or
Only play this for a post fill 9-12

Material in A or B part markers should play in their section.
Remember to hit the Update as you go.

The more playing you have the better as it's less likely to over transpose things, there are 12key RealTrack but it depends how keen you are.
You just need to experiment with it and if you are happy with the results use it or upload it.

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UserTrack Settings F5

Bar # and Chorus # indicate the current song location. This is the bar where the settings will be applied, but you can apply them to a range of bars if you enter a list (3,5,7) and/or range (10-20, 25-28) of bars, separated by commas, in the edit control box under Enter a Range of Bars.

Press the [Update] button to save the settings.

Don’t include this bar in the UserTracks excludes the current bar (or range of bars, if set) from the UserTracks and will never be used.

Ending starts on this bar (4 bar ending) sets the current bar as the first bar of the 4 bar ending.

Don’t start a new Riff on this bar
prevents the current bar from being used as the start of a new riff. This is useful if the bar has sustained notes carried over from the previous bar and no new attack of a note.

Don’t End a Riff at the start of this bar means that the current bar (or list of bars in the edit control) will not be used to end a riff at the start of the bar. This is useful if the previous bar doesn’t have a defined ending for a riff.

Only choose this bar if the key matches allows the bar to be used only if the user’s song key matches the key of this song. So, if this is the IV chord, it would only get chosen if it will be the IV chord in the user’s song.

This bar is simple sets the bar to use simple riffs.

Only Play this for a fill sets the current bar as a fill, which will only be played where the user’s song is also a fill.

Only play this for a post fill (after a fill) sets the current bar (or list of bars in the edit control) to only play after a fill in the user’s song.

Odd/Even must match sets the current bar to play in the user’s song only if the odd/even status (bars after a part marker) matches the user’s song.

Always play this bar if it matches means that the current bar (or list of bars) will always get played, as long as its chord and other settings are compatible. It overrides any other riffs that match.

Once is enough (don’t repeat this) sets the bar to only be played once in a song. Use for an unusual riff that you don’t want repeated.

Bar/Beat Markers
You can set the location of the bar or beat markers if the riff doesn’t start on the bar or beat. You can set the location in ms offset, and also set the riff marker type, begin and/or end. The offsets can be set visually in the Audio Edit window.
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ReaTrak Biab & Plugin in Reaper
Free Biab Chord Picker Tool