Connecting the Mix Bus Link dsub from an X-Rack to the X-Rack Expansion socket on the rear of the X-desk cascades the Mix Bus and Solo from an X-Rack summing system into the X-Desk.
You must not have an XR622 Master module fitted in the X-Rack, as the X-Desk acts as the master summing bus.
The cable must be a pin-to-pin cable. This is not the same as the Tascam standard.
The pinout tables for these connectors are provided in their respective guides: