[xhevahir]

This uses accompaniment styles from Yamaha arranger keyboards, which have been around for ages. It's a much simpler format (really just a special kind of MIDI file) than the BIAB one, and more open. This project has details: https://github.com/bures/sff2-tools . There's a site that's popular with users of Yamaha keyboards, www.jososoft.dk , that has tons of styles in various genres.

I messed around with JJazzLab a few months ago, and, while I did encounter some annoying MIDI problems involving the drum patches, I liked it enough that I wrote myself a script to import a song from another program, Impro-Visor, for auditioning in JJazzLab. It's a lot easier to use than Band in a Box.