The Arca 6x9 uses an adapter plate that fits in the rear frame and holds the film or digital back. Different film backs, by different manufacturers such as Horseman, need different adapter plates, and the same is true of digital backs. You would need an adapter plate that is specific to the mount of back you are using on it such as Contax, Mamiya, or Hasselblad H/V. The plate is specific to the mount, not the back manufacturer so a Contax fit plate will allow you to use any Contax mount digital back, from Phase to Leaf to Sinar etc.
The adapter plates are made by Arca themselves so you would need to order one from them, or via your dealer.
Keep in mind this is a fixed mounting solution, so no sliding function as seen in other options. The benefit being closer lens mounting for wides. The disadvantage being you have to take the back with plate off the camera to use a ground glass screen if you like to compose that way.
The other issue is availability. I've been trying to get a Contax mounting plate for my Arca 6x9 for seven months now with little success. I hear of others who have managed though so it is possible.
Hope that helps.