The ARC USB is a USB 1.1 (UAC1) MIDI remote control, natively compatible to Windows and Mac OS X. As soon as it is present in the operating system TotaIMix FX will detect it and communicate with it, without disturbing other remote controls. Therefore it is also not necessary to switch the ARC USB on or off within TotalMix FX - it will just work.
And because it communicates with TotalMix FX (via USB) it is compatible to all RME audio interfaces that are detected and supported by TotalMix FX. This goes back to RME interfaces released 2001!

As soon as the ARC USB is connected to a computer any button press or wheel turning will cause the respective action performed by TotalMix FX - and therefore also by the RME audio interface.

With the UFX+ and UFX II the ARC USB can also be connected directly to these devices, currently the only RME interfaces having an internal USB 1.1 host with matching connector. This way of connecting is handy in stand-alone mode, where the ARC USB is also supported by the UFX+ and UFX II.

The ARC USB is bus powered, but draws only little current. Also it uses USB 1.1 as bus standard, which these days is uncritical due to its lower frequency and speed demands. USB 2 cables with 5 meter to 10 meter length, and cheap USB 2 cable extensions that usually make a bus-powered interface fail will work perfectly for the ARC USB. Still at higher lengths RME recommends to use active USB 2 cables (also called extension or repeater cables), which are not expensive and work over lengths of more than 20 meters.

The ARC USB can control several interfaces simultaneously, which happens automatically as the Key Commands dialog in TotalMix FX is specific to the currently selected audio interface. Just assign only the functions that are desired for each respective interface. After that the ARC USB keys and the encoder wheel remote the respective interface where they have been assigned to, without the need to further switch or select anything in normal operation.