Sorry if this is an elementary question, but I've just spent a couple of hours wading through the MODX manuals without success.

Some performances allow the part volume to be overridden by using the control sliders, but some do not. When it's allowed, the small triangle in the performance display (next to the volume level) turns white and the grey vertical bar moves as you move the slider; when it's not allowed, it stays green and the grey bar stays where it was. How do I enable this?

It's obviously very convenient for remixing performances on the fly, so I'm not sure why it's even configurable.