What message do control surfaces send to show that a slider is currently being used.

I have a behringer bcf2000 and am wanting to experiment using other buttons on the device to emulate touch controls that would allow me to use the device in touch mode.

What (midi) message does the controller send to ardour to let ardour know that that particular slider is being touched?

There is no standard for this. In the Logic control it is defined(I don’t have it in front of me at the moment to look it up) but there is nothing in MIDI to my knowledge that defines a standard TOUCH message.