Touché (SE) is a class-compliant USB MIDI device and will be recognized as such by the iPad. However, be aware that there is no iPad version of Touché's companion app Lié. In order to play the factory sounds and configure the device in-depth, you will need to use the standalone app or the VST/AU plugin on a Mac or PC.
You can connect Touché (SE) to an iPad via USB. If your iPad has a Lightning connector, then it will require a Camera Connection Kit (optional purchase). Depending on your iPad model it is possible that it doesn't deliver enough current for Touché (SE) to work. If your device's LEDs remain dark or if Touché (SE) doesn't show up in the MIDI input list or works unreliable, please power Touché (SE) via an external USB power adapter. Because Touché (SE) only has a single USB B port, a Y cable will be needed to split power and data from each other. This adapter is included with the classic Touché, but is a separate purchase in case of Touché SE. You can get it here: Ground Loop Adaptor in the EE webshop
Once a secure connection is established, you will be able to control any app that accepts being remotely controlled via MIDI CC messages (and pitch bend). It works in the same way as when mapping the faders or knobs of a generic USB MIDI controller. This third-party video gives an example of how you could use a Touché with your iPad:
By default, Touché (SE) sends MIDI CCs #16/17/18/19 on its four dimensions of movement. If you wish to customize the MIDI CCs being sent, you will need to use the companion app Lié on your Mac or PC. For the classic Touché, you would create a hardware preset and transfer it into Touché's internal memory using Memory View. For Touché SE, you would adapt the single Standalone memory it has by altering its MIDI Setup. Read here to learn more about the different capabilities of Touché and Touché SE when they are in Standalone Mode.