Extra CC and hold can be applied to Knobs, Sliders, and X-Y Pads. It establishes that a toggle button will be momentarily ON (sending its max value) while the control (knob, etc.) is being touched.
When you let go of the control, the Extra CC and Hold button shuts off.
If you press the Extra CC and Hold Button while you are touching the control, it will not shut off until you touch the Button again.
If the Extra CC and Hold Button was on before you touched the control, it doesn’t change when you touch the control.
In the current version of MIDI Designer, it looks like this (for iPhone/iPod touch):
