If you had two singers on stage standing together the Sound Space would be narrow ( a low number), if they stood wide apart the Sound Space would be broad ( a high number).
I figured it might be something like that. Using the term Stereo-Width or even just Width would have avoided the confusion with the Randomizer Space option and the Reverb Space preset.
I note it can quickly drive the signal into distortion requiring lower overall track levels to compensate.