Column recording settings
You will see this when selecting a Column cell.
Basics ⊖
Origin
This decides from where to capture audio input during recording.
Track audio output
Records the audio output of a track, post-FX. This is great for recording the output of virtual instruments.
This feature uses REAPER loopback audio hardware channels under the hood. That means it only works if you have at least 2 loopback channels enabled. You can enable them in "REAPER Preferences → Audio → Virtual loopback audio hardware channels".
You should choose a recording track that’s different from the playback track. Otherwise, other clips playing on the playback track will be recorded as well. Probably not what you want ;)