It's described here (version 9 manual): https://steinberg.help/cubase_pro_artis ... out_r.html
But, I can't find the option at all on the Cubase Artist 12 manual: https://steinberg.help/cubase_artist/v1 ... own_r.html
Also, check out the actual description in the Cubase 9 manual:
Are we talking about the same thing? I'm talking about playing a VSTi, over your MIDI keyboard, for example, and recording the audio output of that VSTi.Real-Time Export
Allows you to export the mixdown file in realtime. This takes at least the same time as regular playback. Some VST plug-ins, external instruments, and effects require real-time export in order to have enough time to update correctly during the mixdown. Consult the plug-in manufacturers for further information.