Mac and Blake - thanks for the replies.
Mac: I hadn't thought about the fact that for RealTracks, the Cadd9 would most likely not have been played by the artist when recording a genre for which that chord would not be typical. When I say "I am an all RealTracks user", I mean that I attempt to build every tune using RealTracks. But I do occasionally use some MIDI voices (usually acoustic or electric bass or acoustic piano) to "fill in" parts of a tune where I can't direct RealTracks to play someting precisely as I want it, and I also use MIDI in tunes where I want strings. But since most of my BiaB work is to produce jazz backup tracks for live performance (my wife sings and I play a Yamaha WX-5), I find that that RealTracks gives me the realistic sound I want 99% of the time. By the way, RealTrack #1998 was released with BiaB 2013. It's Miles doing a great funky jazz sound on an acoustic piano!

Blake: Thanks for the answer re: the fact that Cadd9 would only work in RealTracks released in 2012.5 or later. I tried it out with a couple of the jazz piano and guitar tracks that were released in 2013 for which a Cadd9 would be considered an approriate chord (1987 and 1991) and it worked great.