The Novation Launch Control XL and Launch Control mini are MIDI control surfaces that connect over a MIDI interface. They feature a bunch of buttons and knobs, and on the XL also sliders. We often use them for online adjustment of parameters, or to toggle some algorithm on or off. See also the documentation on manual control.
As an alternative to a hardware controller, you can also use the inputcontrol module, which provides a configurable graphical user interface on the computer screen.
To use the Launch Control XL connected directly (without powered USB hub) with the Raspberry Pi, you must first switch it to low-power mode. To do this hold down both the User and Factory Template buttons and insert the USB cable. Release the buttons and press Record Arm. Finally press the right arrow button.