It says holdover, so I assume it is the button you can press and hold that acts like a DSD pedal if you cannot keep your foot on it.
That's what I thought. But shouldn't it say like "DSD Holdover' instead of 'Cab Holdover' because it kinda sounds like 'Cab Change' switch... But I again... I took over this cab without pressing this button so that possibility is ruled out...
