I'm new to BIAB, but it seems to me that RealTracks are the sounds (timbre) that are played and Styles are the rhythms/notes (notation) according to which those sounds are played. RealTracks determine how the instruments sound while Styles determine when each note is played.

I've collected all the RealTracks but I now see that the XtraPak Styles are where those RealTracks are leveraged and made to shine.