I'm pretty new to using Rapture Pro, and I currently use it with reaper. I know that there is an option to use Rapture Pro as a multitimbral synth, but I'm not too sure how that works. I've referenced the user manual several times and I'm still not sure how route these tracks. Please help!

When Multitimbral mode is enabled in the options drop down, the elements in the synth are hard-coded to respond to MIDI channels 1 through 6 respectively. In multitimbral mode the typical setup would be up to 6 MIDI tracks set to MIDI channels 1 through 6 and a single stereo or two mono audio tracks to play the result.

This is particularly cool if you have a guitar synth like the Roland GR20 which can send each string on a separate channel.