You use the soundfonts inside the RCvst or your own soundfonts you add to the list.
The best way is Load KV Element into Biab then drag the RCvst3 into Element, if you are running Biab as admin you will need to drag it in using a file explorer running as admin (like Explorer++).
This way you can route midi channels out of RCvst to Virtual Instruments.
Make sure you have the EXT button top left in Element on to sync.
RapidComposer is always having sales, see the News page.

And I just looked there and:
November 16, 2020: RapidComposer v4.0 released with 25% intro offer!
You can upgrade anytime to the full version just paying the difference.
http://www.musicdevelopments.com/news.html

If you get it, get the Beta version after and go to to the Beta forum, if you suggest features they are usually implemented very quick.