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@Muzamil > It depends on how you're using rekordbox.
If you're using rekordbox (Export mode):
You can link the computer and the players together via ethernet cable (ProDJ Link), but you'll need to use an ethernet switch or hub. I don't recommend a router as they can have traffic management systems and interfere with the communication. There are other troubleshooting tips here in case you can't get the players communicating with your computer over the ProDJ Link.
As rekordbox, in Export mode, doesn't have a recording function, you'd need to use a 3rd party software to record the audio back from your mixer. You could do this with through the recording input from the Xone:42 connected via USB.
If you're using rekordbox dj (Performance mode):
This requires that you've purchased a rekordbox dj license.
You have two choices here - either use the XDJs as the audio interfaces (both connected via USB), and then the audio outputs from the XDJs to the mixer's line input. The other option is to use the mixer as the audio device; the Xone:42 features a built-in audio interface. Either way, you would need to have all 3 devices connected by USB, and configure the rekordbox audio preference for external mixer mode.
As above, the Xone:42 also has a recording input, which you would select within the rekordbox audio preferences as the recording source.